branch: make create_branch accept a merge base rev

When we ran something like

    $ git checkout -b test master...

it would fail with the message

    fatal: Not a valid object name: 'master...'.

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>

branch.c

@@ -268,7 +268,7 @@ void create_branch(struct repository *r,
 	}
 
 	real_ref = NULL;
-	if (get_oid(start_name, &oid)) {
+	if (get_oid_mb(start_name, &oid)) {
 		if (explicit_tracking) {
 			if (advice_set_upstream_failure) {
 				error(_(upstream_missing), start_name);