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

124371c5a1

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

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

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"