patch 9.1.0099: Not able to use diff() with 'diffexpr'

Problem:  Not able to use diff() with 'diffexpr'
          (rickhowe, after v9.1.0096)
Solution: Use a default context length of 0, update diff() help text,
          add a test for using diff() with 'diffexpr'
          (Yegappan Lakshmanan)

closes: #14013

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 d37a9a2..8e7d5ce 100644
--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
Binary files differ