rebase-i-p: use HEAD for updating the ref instead of mapping OLDHEAD

If OLDHEAD was reordered in the todo, and its mapped NEWHEAD was used to set the
ref, commits reordered after OLDHEAD in the todo would should up as un-committed
changes.

Signed-off-by: Stephen Haberman <stephen@exigencecorp.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>