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/src/testdir/test_syntax.vim b/src/testdir/test_syntax.vim
index 8c56730..aeb2c51 100644
--- a/src/testdir/test_syntax.vim
+++ b/src/testdir/test_syntax.vim
@@ -193,14 +193,14 @@
   " Check that clearing "Aap" avoids it showing up before Boolean.
   hi Aap ctermfg=blue
   call feedkeys(":syn list \<C-A>\<C-B>\"\<CR>", 'tx')
-  call assert_match('^"syn list Aap Boolean Character ', @:)
+  call assert_match('^"syn list Aap Added Boolean Changed Character ', @:)
   hi clear Aap
 
   call feedkeys(":syn list \<C-A>\<C-B>\"\<CR>", 'tx')
-  call assert_match('^"syn list Boolean Character ', @:)
+  call assert_match('^"syn list Added Boolean Changed Character ', @:)
 
   call feedkeys(":syn match \<C-A>\<C-B>\"\<CR>", 'tx')
-  call assert_match('^"syn match Boolean Character ', @:)
+  call assert_match('^"syn match Added Boolean Changed Character ', @:)
 
   syn cluster Aax contains=Aap
   call feedkeys(":syn list @A\<C-A>\<C-B>\"\<CR>", 'tx')
diff --git a/src/version.c b/src/version.c
index 9a4106d..2e178f3 100644
--- a/src/version.c
+++ b/src/version.c
@@ -705,6 +705,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    16,
+/**/
     15,
 /**/
     14,