diffstat: make --git work properly on renames (issue6025)

Authored by navaneeth.suresh.

Description

diffstat: make --git work properly on renames (issue6025)

$ hg diff --stat --git shows only the new filename on renames.
I added the old filename also to the output to make it identical
with the output of $ git diff --stat.

Differential Revision: https://phab.mercurial-scm.org/D5628

Details