rebase: pass correct arguments to post-checkout hook

rebase: pass correct arguments to post-checkout hook

If a rebase started with "rebase [--apply|--merge] <upstream> <branch>"
detects that <upstream> is an ancestor of <branch> then it fast-forwards
and checks out <branch>. Unfortunately in that case it passed the null
oid as the first argument to the post-checkout hook rather than the oid
of HEAD.

A side effect of this change is that the call to update_ref() which
updates HEAD now always receives the old value of HEAD. This provides
protection against another process updating HEAD during the checkout.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>