runtime(diff): Update default links (#13776)

Problem: Current default links for `diffAdded`, `diffChanged`, and
              `diffRemoved` do not address the diff nature of the filetype.
Solution: Use `DiffAdd`, `DiffChange`, and `DiffDelete`.

closes: #13759

Signed-off-by: Evgeni Chasnovski <evgeni.chasnovski@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

Author: echasnovski

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		Special
-hi def link diffChanged		PreProc
-hi def link diffAdded		Identifier
+hi def link diffRemoved		DiffDelete
+hi def link diffChanged		DiffChange
+hi def link diffAdded		DiffAdd
 hi def link diffLine		Statement
 hi def link diffSubname		PreProc
 hi def link diffComment		Comment