patch 9.1.1557: not possible to anchor specific lines in difff mode

Problem:  not possible to anchor specific lines in difff mode
Solution: Add support for the anchoring lines in diff mode using the
          'diffanchor' option (Yee Cheng Chin).

Adds support for anchoring specific lines to each other while viewing a
diff. While lines are anchored, they are guaranteed to be aligned to
each other in a diff view, allowing the user to control and inform the
diff algorithm what the desired alignment is. Internally, this is done
by splitting up the buffer at each anchor and run the diff algorithm on
each split section separately, and then merge the results back for a
logically consistent diff result.

To do this, add a new "diffanchors" option that takes a list of
`{address}`, and a new "diffopt" option value "anchor". Each address
specified will be an anchor, and the user can choose to use any type of
address, including marks, line numbers, or pattern search. Anchors are
sorted by line number in each file, and it's possible to have multiple
anchors on the same line (this is useful when doing multi-buffer diff).
Update documentation to provide examples.

This is similar to Git diff's `--anchored` flag. Other diff tools like
Meld/Araxis Merge also have similar features (called "synchronization
points" or "synchronization links"). We are not using Git/Xdiff's
`--anchored` implementation here because it has a very limited API
(it requires usage of the Patience algorithm, and can only anchor
unique lines that are the same across both files).

Because the user could anchor anywhere, diff anchors could result in
adjacent diff blocks (one block is directly touching another without a
gap), if there is a change right above the anchor point. We don't want
to merge these diff blocks because we want to line up the change at the
anchor. Adjacent diff blocks were first allowed when linematch was
added, but the existing code had a lot of branched paths where
line-matched diff blocks were handled differently. As a part of this
change, refactor them to have a more unified code path that is
generalized enough to handle adjacent diff blocks correctly and without
needing to carve in exceptions all over the place.

closes: #17615

Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/vim.vim

@@ -2,7 +2,7 @@
 " Language:	   Vim script
 " Maintainer:	   Hirohito Higashi <h.east.727 ATMARK gmail.com>
 "	   Doug Kearns <dougkearns@gmail.com>
-" Last Change:	   2025 Jul 14
+" Last Change:	   2025 Jul 16
 " Former Maintainer: Charles E. Campbell
 
 " DO NOT CHANGE DIRECTLY.
@@ -52,14 +52,14 @@ syn keyword vimStdPlugin contained	Arguments Asm Break Cfilter Clear Continue Di
 " vimOptions are caught only when contained in a vimSet {{{2
 " GEN_SYN_VIM: vimOption normal, START_STR='syn keyword vimOption contained', END_STR='skipwhite nextgroup=vimSetEqual,vimSetMod'
 syn keyword vimOption contained al aleph ari allowrevins ambw ambiwidth arab arabic arshape arabicshape acd autochdir ai autoindent ar autoread asd autoshelldir aw autowrite awa autowriteall bg background bs backspace bk backup bkc backupcopy bdir backupdir bex backupext bsk backupskip bdlay balloondelay beval ballooneval bevalterm balloonevalterm bexpr balloonexpr bo belloff bin binary bomb brk breakat bri breakindent briopt breakindentopt bsdir browsedir bh bufhidden bl buflisted bt buftype cmp casemap cdh cdhome cd cdpath cedit ccv charconvert chi chistory cin cindent cink cinkeys cino cinoptions cinsd cinscopedecls cinw cinwords cb clipboard cpm clipmethod ch cmdheight cwh cmdwinheight cc colorcolumn co columns com comments cms commentstring cp compatible skipwhite nextgroup=vimSetEqual,vimSetMod
-syn keyword vimOption contained cpt complete cfu completefunc cfc completefuzzycollect cia completeitemalign cot completeopt cpp completepopup csl completeslash cocu concealcursor cole conceallevel cf confirm ci copyindent cpo cpoptions cm cryptmethod cspc cscopepathcomp csprg cscopeprg csqf cscopequickfix csre cscoperelative cst cscopetag csto cscopetagorder csverb cscopeverbose crb cursorbind cuc cursorcolumn cul cursorline culopt cursorlineopt debug def define deco delcombine dict dictionary diff dex diffexpr dip diffopt dg digraph dir directory dy display ead eadirection ed edcompatible emo emoji enc encoding eof endoffile eol endofline ea equalalways ep equalprg eb errorbells ef errorfile efm errorformat ek esckeys ei eventignore eiw eventignorewin et expandtab skipwhite nextgroup=vimSetEqual,vimSetMod
-syn keyword vimOption contained ex exrc fenc fileencoding fencs fileencodings ff fileformat ffs fileformats fic fileignorecase ft filetype fcs fillchars ffu findfunc fixeol fixendofline fcl foldclose fdc foldcolumn fen foldenable fde foldexpr fdi foldignore fdl foldlevel fdls foldlevelstart fmr foldmarker fdm foldmethod fml foldminlines fdn foldnestmax fdo foldopen fdt foldtext fex formatexpr flp formatlistpat fo formatoptions fp formatprg fs fsync gd gdefault gfm grepformat gp grepprg gcr guicursor gfn guifont gfs guifontset gfw guifontwide ghr guiheadroom gli guiligatures go guioptions guipty gtl guitablabel gtt guitabtooltip hf helpfile hh helpheight hlg helplang hid hidden hl highlight hi history hk hkmap hkp hkmapp hls hlsearch icon iconstring ic ignorecase skipwhite nextgroup=vimSetEqual,vimSetMod
-syn keyword vimOption contained imaf imactivatefunc imak imactivatekey imc imcmdline imd imdisable imi iminsert ims imsearch imsf imstatusfunc imst imstyle inc include inex includeexpr is incsearch inde indentexpr indk indentkeys inf infercase im insertmode ise isexpand isf isfname isi isident isk iskeyword isp isprint js joinspaces jop jumpoptions key kmp keymap km keymodel kpc keyprotocol kp keywordprg lmap langmap lm langmenu lnr langnoremap lrm langremap ls laststatus lz lazyredraw lhi lhistory lbr linebreak lines lsp linespace lisp lop lispoptions lw lispwords list lcs listchars lpl loadplugins luadll magic mef makeef menc makeencoding mp makeprg mps matchpairs mat matchtime mco maxcombine mfd maxfuncdepth mmd maxmapdepth mm maxmem mmp maxmempattern mmt maxmemtot skipwhite nextgroup=vimSetEqual,vimSetMod
-syn keyword vimOption contained msc maxsearchcount mis menuitems mopt messagesopt msm mkspellmem ml modeline mle modelineexpr mls modelines ma modifiable mod modified more mouse mousef mousefocus mh mousehide mousem mousemodel mousemev mousemoveevent mouses mouseshape mouset mousetime mzq mzquantum mzschemedll mzschemegcdll nf nrformats nu number nuw numberwidth ofu omnifunc odev opendevice opfunc operatorfunc pp packpath para paragraphs paste pt pastetoggle pex patchexpr pm patchmode pa path perldll pi preserveindent pvh previewheight pvp previewpopup pvw previewwindow pdev printdevice penc printencoding pexpr printexpr pfn printfont pheader printheader pmbcs printmbcharset pmbfn printmbfont popt printoptions prompt ph pumheight pmw pummaxwidth pw pumwidth pythondll skipwhite nextgroup=vimSetEqual,vimSetMod
-syn keyword vimOption contained pythonhome pythonthreedll pythonthreehome pyx pyxversion qftf quickfixtextfunc qe quoteescape ro readonly rdt redrawtime re regexpengine rnu relativenumber remap rop renderoptions report rs restorescreen ri revins rl rightleft rlc rightleftcmd rubydll ru ruler ruf rulerformat rtp runtimepath scr scroll scb scrollbind scf scrollfocus sj scrolljump so scrolloff sbo scrollopt sect sections secure sel selection slm selectmode ssop sessionoptions sh shell shcf shellcmdflag sp shellpipe shq shellquote srr shellredir ssl shellslash stmp shelltemp st shelltype sxe shellxescape sxq shellxquote sr shiftround sw shiftwidth shm shortmess sn shortname sbr showbreak sc showcmd sloc showcmdloc sft showfulltag sm showmatch smd showmode stal showtabline skipwhite nextgroup=vimSetEqual,vimSetMod
-syn keyword vimOption contained stpl showtabpanel ss sidescroll siso sidescrolloff scl signcolumn scs smartcase si smartindent sta smarttab sms smoothscroll sts softtabstop spell spc spellcapcheck spf spellfile spl spelllang spo spelloptions sps spellsuggest sb splitbelow spk splitkeep spr splitright sol startofline stl statusline su suffixes sua suffixesadd swf swapfile sws swapsync swb switchbuf smc synmaxcol syn syntax tcl tabclose tal tabline tpm tabpagemax tpl tabpanel tplo tabpanelopt ts tabstop tbs tagbsearch tc tagcase tfu tagfunc tl taglength tr tagrelative tag tags tgst tagstack tcldll term tbidi termbidi tenc termencoding tgc termguicolors twk termwinkey twsl termwinscroll tws termwinsize twt termwintype terse ta textauto tx textmode tw textwidth tsr thesaurus skipwhite nextgroup=vimSetEqual,vimSetMod
-syn keyword vimOption contained tsrfu thesaurusfunc top tildeop to timeout tm timeoutlen title titlelen titleold titlestring tb toolbar tbis toolbariconsize ttimeout ttm ttimeoutlen tbi ttybuiltin tf ttyfast ttym ttymouse tsl ttyscroll tty ttytype udir undodir udf undofile ul undolevels ur undoreload uc updatecount ut updatetime vsts varsofttabstop vts vartabstop vbs verbose vfile verbosefile vdir viewdir vop viewoptions vi viminfo vif viminfofile ve virtualedit vb visualbell warn wiv weirdinvert ww whichwrap wc wildchar wcm wildcharm wig wildignore wic wildignorecase wmnu wildmenu wim wildmode wop wildoptions wak winaltkeys wcr wincolor wi window wfb winfixbuf wfh winfixheight wfw winfixwidth wh winheight wmh winminheight wmw winminwidth winptydll wiw winwidth skipwhite nextgroup=vimSetEqual,vimSetMod
-syn keyword vimOption contained wse wlseat wst wlsteal wtm wltimeoutlen wrap wm wrapmargin ws wrapscan write wa writeany wb writebackup wd writedelay xtermcodes skipwhite nextgroup=vimSetEqual,vimSetMod
+syn keyword vimOption contained cpt complete cfu completefunc cfc completefuzzycollect cia completeitemalign cot completeopt cpp completepopup csl completeslash cocu concealcursor cole conceallevel cf confirm ci copyindent cpo cpoptions cm cryptmethod cspc cscopepathcomp csprg cscopeprg csqf cscopequickfix csre cscoperelative cst cscopetag csto cscopetagorder csverb cscopeverbose crb cursorbind cuc cursorcolumn cul cursorline culopt cursorlineopt debug def define deco delcombine dict dictionary diff dia diffanchors dex diffexpr dip diffopt dg digraph dir directory dy display ead eadirection ed edcompatible emo emoji enc encoding eof endoffile eol endofline ea equalalways ep equalprg eb errorbells ef errorfile efm errorformat ek esckeys ei eventignore eiw eventignorewin skipwhite nextgroup=vimSetEqual,vimSetMod
+syn keyword vimOption contained et expandtab ex exrc fenc fileencoding fencs fileencodings ff fileformat ffs fileformats fic fileignorecase ft filetype fcs fillchars ffu findfunc fixeol fixendofline fcl foldclose fdc foldcolumn fen foldenable fde foldexpr fdi foldignore fdl foldlevel fdls foldlevelstart fmr foldmarker fdm foldmethod fml foldminlines fdn foldnestmax fdo foldopen fdt foldtext fex formatexpr flp formatlistpat fo formatoptions fp formatprg fs fsync gd gdefault gfm grepformat gp grepprg gcr guicursor gfn guifont gfs guifontset gfw guifontwide ghr guiheadroom gli guiligatures go guioptions guipty gtl guitablabel gtt guitabtooltip hf helpfile hh helpheight hlg helplang hid hidden hl highlight hi history hk hkmap hkp hkmapp hls hlsearch icon iconstring skipwhite nextgroup=vimSetEqual,vimSetMod
+syn keyword vimOption contained ic ignorecase imaf imactivatefunc imak imactivatekey imc imcmdline imd imdisable imi iminsert ims imsearch imsf imstatusfunc imst imstyle inc include inex includeexpr is incsearch inde indentexpr indk indentkeys inf infercase im insertmode ise isexpand isf isfname isi isident isk iskeyword isp isprint js joinspaces jop jumpoptions key kmp keymap km keymodel kpc keyprotocol kp keywordprg lmap langmap lm langmenu lnr langnoremap lrm langremap ls laststatus lz lazyredraw lhi lhistory lbr linebreak lines lsp linespace lisp lop lispoptions lw lispwords list lcs listchars lpl loadplugins luadll magic mef makeef menc makeencoding mp makeprg mps matchpairs mat matchtime mco maxcombine mfd maxfuncdepth mmd maxmapdepth mm maxmem mmp maxmempattern skipwhite nextgroup=vimSetEqual,vimSetMod
+syn keyword vimOption contained mmt maxmemtot msc maxsearchcount mis menuitems mopt messagesopt msm mkspellmem ml modeline mle modelineexpr mls modelines ma modifiable mod modified more mouse mousef mousefocus mh mousehide mousem mousemodel mousemev mousemoveevent mouses mouseshape mouset mousetime mzq mzquantum mzschemedll mzschemegcdll nf nrformats nu number nuw numberwidth ofu omnifunc odev opendevice opfunc operatorfunc pp packpath para paragraphs paste pt pastetoggle pex patchexpr pm patchmode pa path perldll pi preserveindent pvh previewheight pvp previewpopup pvw previewwindow pdev printdevice penc printencoding pexpr printexpr pfn printfont pheader printheader pmbcs printmbcharset pmbfn printmbfont popt printoptions prompt ph pumheight pmw pummaxwidth skipwhite nextgroup=vimSetEqual,vimSetMod
+syn keyword vimOption contained pw pumwidth pythondll pythonhome pythonthreedll pythonthreehome pyx pyxversion qftf quickfixtextfunc qe quoteescape ro readonly rdt redrawtime re regexpengine rnu relativenumber remap rop renderoptions report rs restorescreen ri revins rl rightleft rlc rightleftcmd rubydll ru ruler ruf rulerformat rtp runtimepath scr scroll scb scrollbind scf scrollfocus sj scrolljump so scrolloff sbo scrollopt sect sections secure sel selection slm selectmode ssop sessionoptions sh shell shcf shellcmdflag sp shellpipe shq shellquote srr shellredir ssl shellslash stmp shelltemp st shelltype sxe shellxescape sxq shellxquote sr shiftround sw shiftwidth shm shortmess sn shortname sbr showbreak sc showcmd sloc showcmdloc sft showfulltag sm showmatch skipwhite nextgroup=vimSetEqual,vimSetMod
+syn keyword vimOption contained smd showmode stal showtabline stpl showtabpanel ss sidescroll siso sidescrolloff scl signcolumn scs smartcase si smartindent sta smarttab sms smoothscroll sts softtabstop spell spc spellcapcheck spf spellfile spl spelllang spo spelloptions sps spellsuggest sb splitbelow spk splitkeep spr splitright sol startofline stl statusline su suffixes sua suffixesadd swf swapfile sws swapsync swb switchbuf smc synmaxcol syn syntax tcl tabclose tal tabline tpm tabpagemax tpl tabpanel tplo tabpanelopt ts tabstop tbs tagbsearch tc tagcase tfu tagfunc tl taglength tr tagrelative tag tags tgst tagstack tcldll term tbidi termbidi tenc termencoding tgc termguicolors twk termwinkey twsl termwinscroll tws termwinsize twt termwintype terse ta textauto skipwhite nextgroup=vimSetEqual,vimSetMod
+syn keyword vimOption contained tx textmode tw textwidth tsr thesaurus tsrfu thesaurusfunc top tildeop to timeout tm timeoutlen title titlelen titleold titlestring tb toolbar tbis toolbariconsize ttimeout ttm ttimeoutlen tbi ttybuiltin tf ttyfast ttym ttymouse tsl ttyscroll tty ttytype udir undodir udf undofile ul undolevels ur undoreload uc updatecount ut updatetime vsts varsofttabstop vts vartabstop vbs verbose vfile verbosefile vdir viewdir vop viewoptions vi viminfo vif viminfofile ve virtualedit vb visualbell warn wiv weirdinvert ww whichwrap wc wildchar wcm wildcharm wig wildignore wic wildignorecase wmnu wildmenu wim wildmode wop wildoptions wak winaltkeys wcr wincolor wi window wfb winfixbuf wfh winfixheight wfw winfixwidth wh winheight wmh winminheight skipwhite nextgroup=vimSetEqual,vimSetMod
+syn keyword vimOption contained wmw winminwidth winptydll wiw winwidth wse wlseat wst wlsteal wtm wltimeoutlen wrap wm wrapmargin ws wrapscan write wa writeany wb writebackup wd writedelay xtermcodes skipwhite nextgroup=vimSetEqual,vimSetMod
 
 " vimOptions: These are the turn-off setting variants {{{2
 " GEN_SYN_VIM: vimOption turn-off, START_STR='syn keyword vimOption contained', END_STR=''
@@ -91,7 +91,7 @@ syn match   vimOption contained	"t_k;"
 " vimOptions: These are the variable names {{{2
 " GEN_SYN_VIM: vimOption normal variable,           START_STR='syn keyword vimOptionVarName contained', END_STR=''
 syn keyword vimOptionVarName contained al aleph ari allowrevins ambw ambiwidth arab arabic arshape arabicshape acd autochdir ai autoindent ar autoread asd autoshelldir aw autowrite awa autowriteall bg background bs backspace bk backup bkc backupcopy bdir backupdir bex backupext bsk backupskip bdlay balloondelay beval ballooneval bevalterm balloonevalterm bexpr balloonexpr bo belloff bin binary bomb brk breakat bri breakindent briopt breakindentopt bsdir browsedir bh bufhidden bl buflisted bt buftype cmp casemap cdh cdhome cd cdpath cedit ccv charconvert chi chistory cin cindent cink cinkeys cino cinoptions cinsd cinscopedecls cinw cinwords cb clipboard cpm clipmethod ch cmdheight cwh cmdwinheight cc colorcolumn co columns com comments cms commentstring cp compatible
-syn keyword vimOptionVarName contained cpt complete cfu completefunc cfc completefuzzycollect cia completeitemalign cot completeopt cpp completepopup csl completeslash cocu concealcursor cole conceallevel cf confirm ci copyindent cpo cpoptions cm cryptmethod cspc cscopepathcomp csprg cscopeprg csqf cscopequickfix csre cscoperelative cst cscopetag csto cscopetagorder csverb cscopeverbose crb cursorbind cuc cursorcolumn cul cursorline culopt cursorlineopt debug def define deco delcombine dict dictionary diff dex diffexpr dip diffopt dg digraph dir directory dy display ead eadirection ed edcompatible emo emoji enc encoding eof endoffile eol endofline ea equalalways ep equalprg eb errorbells ef errorfile efm errorformat ek esckeys ei eventignore eiw eventignorewin
+syn keyword vimOptionVarName contained cpt complete cfu completefunc cfc completefuzzycollect cia completeitemalign cot completeopt cpp completepopup csl completeslash cocu concealcursor cole conceallevel cf confirm ci copyindent cpo cpoptions cm cryptmethod cspc cscopepathcomp csprg cscopeprg csqf cscopequickfix csre cscoperelative cst cscopetag csto cscopetagorder csverb cscopeverbose crb cursorbind cuc cursorcolumn cul cursorline culopt cursorlineopt debug def define deco delcombine dict dictionary diff dia diffanchors dex diffexpr dip diffopt dg digraph dir directory dy display ead eadirection ed edcompatible emo emoji enc encoding eof endoffile eol endofline ea equalalways ep equalprg eb errorbells ef errorfile efm errorformat ek esckeys ei eventignore eiw eventignorewin
 syn keyword vimOptionVarName contained et expandtab ex exrc fenc fileencoding fencs fileencodings ff fileformat ffs fileformats fic fileignorecase ft filetype fcs fillchars ffu findfunc fixeol fixendofline fcl foldclose fdc foldcolumn fen foldenable fde foldexpr fdi foldignore fdl foldlevel fdls foldlevelstart fmr foldmarker fdm foldmethod fml foldminlines fdn foldnestmax fdo foldopen fdt foldtext fex formatexpr flp formatlistpat fo formatoptions fp formatprg fs fsync gd gdefault gfm grepformat gp grepprg gcr guicursor gfn guifont gfs guifontset gfw guifontwide ghr guiheadroom gli guiligatures go guioptions guipty gtl guitablabel gtt guitabtooltip hf helpfile hh helpheight hlg helplang hid hidden hl highlight hi history hk hkmap hkp hkmapp hls hlsearch icon iconstring
 syn keyword vimOptionVarName contained ic ignorecase imaf imactivatefunc imak imactivatekey imc imcmdline imd imdisable imi iminsert ims imsearch imsf imstatusfunc imst imstyle inc include inex includeexpr is incsearch inde indentexpr indk indentkeys inf infercase im insertmode ise isexpand isf isfname isi isident isk iskeyword isp isprint js joinspaces jop jumpoptions key kmp keymap km keymodel kpc keyprotocol kp keywordprg lmap langmap lm langmenu lnr langnoremap lrm langremap ls laststatus lz lazyredraw lhi lhistory lbr linebreak lines lsp linespace lisp lop lispoptions lw lispwords list lcs listchars lpl loadplugins luadll magic mef makeef menc makeencoding mp makeprg mps matchpairs mat matchtime mco maxcombine mfd maxfuncdepth mmd maxmapdepth mm maxmem mmp maxmempattern
 syn keyword vimOptionVarName contained mmt maxmemtot msc maxsearchcount mis menuitems mopt messagesopt msm mkspellmem ml modeline mle modelineexpr mls modelines ma modifiable mod modified more mouse mousef mousefocus mh mousehide mousem mousemodel mousemev mousemoveevent mouses mouseshape mouset mousetime mzq mzquantum mzschemedll mzschemegcdll nf nrformats nu number nuw numberwidth ofu omnifunc odev opendevice opfunc operatorfunc pp packpath para paragraphs paste pt pastetoggle pex patchexpr pm patchmode pa path perldll pi preserveindent pvh previewheight pvp previewpopup pvw previewwindow pdev printdevice penc printencoding pexpr printexpr pfn printfont pheader printheader pmbcs printmbcharset pmbfn printmbfont popt printoptions prompt ph pumheight pmw pummaxwidth