A merge is not necessarily with a remote branch, it can be with any
commit.

Thanks to Paolo Ciarrocchi for pointing out the problem, and to
Nicolas Pitre for pointing out the fact that a merge is not
necessarily with a branch head.
Signed-off-by: NMatthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>