[PATCH] Resurrect diff-tree-helper -R

Diff-tree-helper take two patch inadvertently dropped the
support of -R option, which is necessary to produce reverse diff
based on diff-cache and diff-files output (diff-tree does not
matter since you can feed two trees in reverse order).  This
patch restores it.

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