rebase: use correct base for --keep-base when a branch is given
--keep-base rebases onto the merge base of the given upstream and the
current HEAD regardless of whether a branch is given. This is contrary
to the documentation and to the option's intended purpose. Instead,
rebase onto the merge base of the given upstream and the given branch.
Signed-off-by: Alex Henrie <alexhenrie24@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>