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 #13776 and doc added in commit b1392be

The links added in #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: #13776
closes #13825

Signed-off-by: Romain Lafourcade <romain.lafourcade@razorfish.fr>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/help.vim

@@ -141,6 +141,10 @@ 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
@@ -216,6 +220,9 @@ 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
 
 if has('textprop') && expand('%:p') =~ '[/\\]doc[/\\]syntax.txt'
   " highlight groups with their respective color