Accept diffs formatted by UnifiedDiffFormatter

GNU diff and mediawiki's internal UnifiedDiffFormatter do not have
the same default formats.  Here we adjust the output of the internal
diff to match gnu diff as is expected by DiscussionParser.
Bug: 41689

Change-Id: Ib83cacab41adfbdfa8e122c0494b266d4caefc83
Merge "Accept diffs formatted by UnifiedDiffFormatter"