refs: update ref transactions to use struct object_id

Update the ref transaction code to use struct object_id.  Remove one
NULL pointer check which was previously inserted around a dereference;
since we now pass a pointer to struct object_id directly through, the
code we're calling handles this for us.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

branch.c

@@ -305,7 +305,7 @@ void create_branch(const char *name, const char *start_name,
 		transaction = ref_transaction_begin(&err);
 		if (!transaction ||
 		    ref_transaction_update(transaction, ref.buf,
-					   oid.hash, forcing ? NULL : null_sha1,
+					   &oid, forcing ? NULL : &null_oid,
 					   0, msg, &err) ||
 		    ref_transaction_commit(transaction, &err))
 			die("%s", err.buf);