patch 8.2.3843: dep3patch files are not recognized

Problem:    Dep3patch files are not recognized.
Solution:   Recognize dep3patch files by their location and content. (James
            McCoy, closes #9367)

runtime/autoload/dist/ft.vim

@@ -811,6 +811,23 @@ func dist#ft#Redif()
   endwhile
 endfunc
 
+" This function is called for all files under */debian/patches/*, make sure not
+" to non-dep3patch files, such as README and other text files.
+func dist#ft#Dep3patch()
+  if expand('%:t') ==# 'series'
+    return
+  endif
+
+  for ln in getline(1, 100)
+    if ln =~# '^\%(Description\|Subject\|Origin\|Bug\|Forwarded\|Author\|From\|Reviewed-by\|Acked-by\|Last-Updated\|Applied-Upstream\):'
+      setf dep3patch
+      return
+    elseif ln =~# '^---'
+      " end of headers found. stop processing
+      return
+    endif
+  endfor
+endfunc
 
 " Restore 'cpoptions'
 let &cpo = s:cpo_save