patch 9.1.0071: Need a diff() Vim script function

patch 9.1.0071: Need a diff() Vim script function

Problem:  Need a diff() Vim script function
Solution: Add the diff() Vim script function using the
          xdiff internal diff library, add support for
          "unified" and "indices" mode.
          (Yegappan Lakshmanan)

fixes: #4241
closes: #12321

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>