patch 9.1.1819: Cannot configure the inner foldlevel indicator

Problem:  Cannot configure the inner foldlevel indicator for the
          foldcolumn
Solution: Add "foldinner" suboption value to the 'fillchar' option
          (Maria José Solano).

closes: #18365

Signed-off-by: Maria José Solano <majosolano99@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/screen.c

@@ -287,6 +287,8 @@ fill_foldcolumn(
 	    symbol = wp->w_fill_chars.foldopen;
 	else if (first_level == 1)
 	    symbol = wp->w_fill_chars.foldsep;
+	else if (wp->w_fill_chars.foldinner != NUL)
+	    symbol = wp->w_fill_chars.foldinner;
 	else if (first_level + i <= 9)
 	    symbol = '0' + first_level + i;
 	else
@@ -4738,6 +4740,7 @@ static struct charstab filltab[] =
     CHARSTAB_ENTRY(&fill_chars.foldopen,    "foldopen"),
     CHARSTAB_ENTRY(&fill_chars.foldclosed,  "foldclose"),
     CHARSTAB_ENTRY(&fill_chars.foldsep,	    "foldsep"),
+    CHARSTAB_ENTRY(&fill_chars.foldinner,   "foldinner"),
     CHARSTAB_ENTRY(&fill_chars.diff,	    "diff"),
     CHARSTAB_ENTRY(&fill_chars.eob,	    "eob"),
     CHARSTAB_ENTRY(&fill_chars.lastline,    "lastline"),
@@ -4856,6 +4859,7 @@ set_chars_option(win_T *wp, char_u *value, int is_listchars, int apply,
 		fill_chars.foldopen = '-';
 		fill_chars.foldclosed = '+';
 		fill_chars.foldsep = '|';
+		fill_chars.foldinner = NUL;
 		fill_chars.diff = '-';
 		fill_chars.eob = '~';
 		fill_chars.lastline = '@';

src/structs.h

@@ -3913,6 +3913,7 @@ typedef struct
     int	foldopen;
     int	foldclosed;
     int	foldsep;
+    int	foldinner;
     int	diff;
     int	eob;
     int	lastline;

src/testdir/test_display.vim

@@ -340,6 +340,32 @@ func Test_fold_fillchars()
         \ ]
   call assert_equal(expected, lines)
 
+  " check setting foldinner
+  set fdc=1 foldmethod=indent foldlevel=10
+  call setline(1, ['one', '	two', '	two', '		three', '		three', 'four'])
+  let lines = ScreenLines([1, 6], 22)
+  let expected = [
+        \ ' one                  ',
+        \ '[        two          ',
+        \ '-        two          ',
+        \ '[                three',
+        \ '2                three',
+        \ ' four                 ',
+        \ ]
+  call assert_equal(expected, lines)
+
+  set fillchars+=foldinner:\ 
+  let lines = ScreenLines([1, 6], 22)
+  let expected = [
+        \ ' one                  ',
+        \ '[        two          ',
+        \ '-        two          ',
+        \ '[                three',
+        \ '                 three',
+        \ ' four                 ',
+        \ ]
+  call assert_equal(expected, lines)
+
   %bw!
   set fillchars& fdc& foldmethod& foldenable&
 endfunc

src/version.c

@@ -729,6 +729,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1819,
 /**/
     1818,
 /**/