[PATCH] show-diff.c: -R option for reverse diff.

[PATCH] show-diff.c: -R option for reverse diff.

This adds -R option to obtain reverse diff.

It may be useful in the merge workflow.  After the base of the working
directory is merged and commited, in the working directory:

    $ read-tree <tree-id-of-merged-tree>
    $ show-diff -R

to re-validate if upstream changes make sense, and/or revert or
conflict with local changes you have in the working files.

Signed-off-by: Junio C Hamano <junkio@cox.net>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>