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>
diff --git a/runtime/syntax/diff.vim b/runtime/syntax/diff.vim
index b5c3f5e..b0a8594 100644
--- a/runtime/syntax/diff.vim
+++ b/runtime/syntax/diff.vim
@@ -378,9 +378,9 @@
 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 b734c2c..2024d38 100644
--- a/runtime/syntax/help.vim
+++ b/runtime/syntax/help.vim
@@ -141,6 +141,10 @@
 
 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 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
diff --git a/runtime/syntax/syncolor.vim b/runtime/syntax/syncolor.vim
index 6cf38cf..52b3a0b 100644
--- a/runtime/syntax/syncolor.vim
+++ b/runtime/syntax/syncolor.vim
@@ -41,6 +41,9 @@
   SynColor Type		term=underline cterm=NONE ctermfg=LightGreen ctermbg=NONE gui=bold guifg=#60ff60 guibg=NONE
   SynColor Underlined	term=underline cterm=underline ctermfg=LightBlue gui=underline guifg=#80a0ff
   SynColor Ignore	term=NONE cterm=NONE ctermfg=black ctermbg=NONE gui=NONE guifg=bg guibg=NONE
+  SynColor Added	term=NONE cterm=NONE ctermfg=Green ctermbg=NONE gui=NONE guifg=LimeGreen guibg=NONE
+  SynColor Changed	term=NONE cterm=NONE ctermfg=Blue ctermbg=NONE gui=NONE guifg=DodgerBlue guibg=NONE
+  SynColor Removed	term=NONE cterm=NONE ctermfg=Red ctermbg=NONE gui=NONE guifg=Red guibg=NONE
 else
   SynColor Comment	term=bold cterm=NONE ctermfg=DarkBlue ctermbg=NONE gui=NONE guifg=Blue guibg=NONE
   SynColor Constant	term=underline cterm=NONE ctermfg=DarkRed ctermbg=NONE gui=NONE guifg=Magenta guibg=NONE
@@ -53,6 +56,9 @@
   SynColor Type		term=underline cterm=NONE ctermfg=DarkGreen ctermbg=NONE gui=bold guifg=SeaGreen guibg=NONE
   SynColor Underlined	term=underline cterm=underline ctermfg=DarkMagenta gui=underline guifg=SlateBlue
   SynColor Ignore	term=NONE cterm=NONE ctermfg=white ctermbg=NONE gui=NONE guifg=bg guibg=NONE
+  SynColor Added	term=NONE cterm=NONE ctermfg=DarkGreen ctermbg=NONE gui=NONE guifg=SeaGreen guibg=NONE
+  SynColor Changed	term=NONE cterm=NONE ctermfg=Blue ctermbg=NONE gui=NONE guifg=DodgerBlue guibg=NONE
+  SynColor Removed	term=NONE cterm=NONE ctermfg=Red ctermbg=NONE gui=NONE guifg=Red guibg=NONE
 endif
 SynColor Error		term=reverse cterm=NONE ctermfg=White ctermbg=Red gui=NONE guifg=White guibg=Red
 SynColor Todo		term=standout cterm=NONE ctermfg=Black ctermbg=Yellow gui=NONE guifg=Blue guibg=Yellow