This was caused by the call to `create_branch` where `start_name` is
expected to be a valid rev. However, git-checkout allows the branch to
be a valid _merge base_ rev (i.e. with a "...") so it was possible for
an invalid rev to be passed in.
Make `create_branch` accept a merge base rev so that this case does not
error out.
As a side-effect, teach git-branch how to handle merge base revs as
well.
Helped-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Denton Liu <liu.denton@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>