"git-diff-tree", "git-diff-files" and "git-diff" can take '-c' or
'--cc' option to produce 'combined diff'. For showing a merge commit
-with "git log -p", this is the default format.
+with "git log -p", this is the default format; you can force showing
+full diff with the '-m' option.
A 'combined diff' format looks like this:
------------