patch 9.1.0096: diff() function uses 'diffexpr'

Problem:  diff() function uses 'diffexpr'
          (rickhowe)
Solution: Make diff() always use internal diff(), add support for
          unified diff context length, sort diff() options in help
          (Yegappan Lakshmanan)

fixes: #13989
closes: #14010

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_diffmode.vim b/src/testdir/test_diffmode.vim
index 2211ba05a..d37a9a2 100644
--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
Binary files differ