Like diff-tree, this patch makes -C option for diff-* brothers
to use only pre-image of modified files as rename/copy detection
by default.  Give --find-copies-harder to use unmodified files
to find copies from as well.

This also fixes "diff-files -C" problem earlier noticed by
Linus.  It was feeding the null sha1 even when the file in the
work tree was known to match what is in the index file.  This
resulted in diff-files showing everything in the project.
Signed-off-by: NJunio C Hamano <junkio@cox.net>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>