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>

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    99,
 /**/
     98,
 /**/