This implements a 3-way diff between the HEAD commit, the state in the
index, and the working directory.  This is like the n-way diff for a
merge, and uses much of the same code.  It is invoked with the -c flag
to git-diff-index, which it already accepted and did nothing with.
Signed-off-by: NPaul Mackerras <paulus@samba.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>