vim-patch:9.1.0016: default diff highlighting is too noisy

Problem:  default diff highlighting is too noisy
Solution: Link diff highlighting groups to new
          Added/Removed/Changed, revert previous change
          (Romain Lafourcade)

Remove diff* links added in vim/vim#13776 and doc added in commit b1392be

The links added in vim/vim#13776 are way too noisy for the contexts in which
the `diff` syntax is applied (git commits, patches, etc.).

This commit:

- removes those links
- adds new default highlighting groups Added, Changed and
  Removed
- links the diff highlighting groups to those new defaults
- removes the doc changes
- adjusts the syntax_completion test for those newly added group
  names

Note: Changes to the default color schemes will be handled separately,
by adding links to those newly created Added/Removed/Changed
highlighting groups.

related: vim/vim#13776
closes vim/vim#13825

https://github.com/vim/vim/commit/124371c5a149a8c0c75c04b6c90ac11e71a0aa97

Co-authored-by: Romain Lafourcade <romain.lafourcade@razorfish.fr>

3 files changed, 15 insertions, 13 deletions

diff --git a/runtime/doc/syntax.txt b/runtime/doc/syntax.txt
index 84121be4d6..8cbb958e35 100644
--- a/runtime/doc/syntax.txt
+++ b/runtime/doc/syntax.txt
@@ -243,6 +243,10 @@ Error		any erroneous construct
 Todo		anything that needs extra attention; mostly the
 		keywords TODO FIXME and XXX
 
+Added		added line in a diff
+Changed		changed line in a diff
+Removed		removed line in a diff
+
 The names marked with * are the preferred groups; the others are minor groups.
 For the preferred groups, the "syntax.vim" file contains default highlighting.
 The minor groups are linked to the preferred groups, so they get the same
@@ -1224,7 +1228,7 @@ To highlight KDE-reserved features, set >
 g:desktop_enable_kde follows g:desktop_enable_nonstd if not supplied
 
 
-DIFF						*diff.vim* *ft-diff-syntax*
+DIFF							*diff.vim*
 
 The diff highlighting normally finds translated headers.  This can be slow if
 there are very long lines in the file.  To disable translations: >
@@ -1233,15 +1237,6 @@ there are very long lines in the file.  To disable translations: >
 
 Also see |diff-slow|.
 
-Since the Vim 9.1 release the diff filetype links the diffAdded,
-diffRemoved and diffChanged highlighting groups to |hl-DiffAdd|,
-|hl-DiffDelete| and |hl-DiffChange| by default.  If you do not want this, you
-can change it to the previous groups like this in your |vimrc| >
-
-	hi link diffRemoved Special
-	hi link diffChanged PreProc
-	hi link diffAdded Identifier
-<
 DIRCOLORS			       *dircolors.vim* *ft-dircolors-syntax*
 
 The dircolors utility highlighting definition has one option.  It exists to
diff --git a/runtime/syntax/diff.vim b/runtime/syntax/diff.vim
index b5c3f5ecbb..b0a8594def 100644
--- a/runtime/syntax/diff.vim
+++ b/runtime/syntax/diff.vim
@@ -378,9 +378,9 @@ hi def link diffBDiffer		Constant
 hi def link diffIsA		Constant
 hi def link diffNoEOL		Constant
 hi def link diffCommon		Constant
-hi def link diffRemoved		DiffDelete
-hi def link diffChanged		DiffChange
-hi def link diffAdded		DiffAdd
+hi def link diffRemoved		Removed
+hi def link diffChanged		Changed
+hi def link diffAdded		Added
 hi def link diffLine		Statement
 hi def link diffSubname		PreProc
 hi def link diffComment		Comment
diff --git a/runtime/syntax/help.vim b/runtime/syntax/help.vim
index f1e650b2fb..fced5e7dd1 100644
--- a/runtime/syntax/help.vim
+++ b/runtime/syntax/help.vim
@@ -136,6 +136,10 @@ syn match helpTodo		"\t[* ]Todo\t\+[a-z].*"
 
 syn match helpURL `\v<(((https?|ftp|gopher)://|(mailto|file|news):)[^' 	<>"]+|(www|web|w3)[a-z0-9_-]*\.[a-z0-9._-]+\.[^' 	<>"]+)[a-zA-Z0-9/]`
 
+syn match helpDiffAdded		"\t[* ]Added\t\+[a-z].*"
+syn match helpDiffChanged	"\t[* ]Changed\t\+[a-z].*"
+syn match helpDiffRemoved	"\t[* ]Removed\t\+[a-z].*"
+
 " Additionally load a language-specific syntax file "help_ab.vim".
 let s:i = match(expand("%"), '\.\a\ax$')
 if s:i > 0
@@ -209,6 +213,9 @@ hi def link helpUnderlined	Underlined
 hi def link helpError		Error
 hi def link helpTodo		Todo
 hi def link helpURL		String
+hi def link helpDiffAdded	Added
+hi def link helpDiffChanged	Changed
+hi def link helpDiffRemoved	Removed
 
 let b:current_syntax = "help"