built-in rebase: set ORIG_HEAD just once, before the rebase

built-in rebase: set ORIG_HEAD just once, before the rebase

Technically, the scripted version set ORIG_HEAD only in two spots (which
really could have been one, because it called `git checkout $onto^0` to
start the rebase and also if it could take a shortcut, and in both cases
it called `git update-ref $orig_head`).

Practically, it *implicitly* reset ORIG_HEAD whenever `git reset --hard`
was called.

However, what we really want is that it is set exactly once, at the
beginning of the rebase.

So let's do that.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>