Teach "am" and "rebase" to mark the original position with ORIG_HEAD

Teach "am" and "rebase" to mark the original position with ORIG_HEAD

"merge" and "reset" leave the original point in history in ORIG_HEAD,
which makes it easy to go back to where you were before you inflict a
major damage to your history and realize that you do not like the result
at all.  These days with reflog, we technically do not need to use
ORIG_HEAD, but it is a handy way nevertheless.

This teaches "am" and "rebase" (all forms --- the vanilla one that uses
"am" as its backend, "-m" variant that cherry-picks, and "--interactive")
to do the same.

The original idea and a partial implementation to do this only for "rebase
-m" was by Brian Gernhardt; this extends on his idea.

Signed-off-by: Junio C Hamano <gitster@pobox.com>