refs: add an update_ref_oid function.

Several places around the codebase want to pass update_ref data from
struct object_id, but update_ref may also be passed NULL pointers.
Instead of checking and dereferencing in every caller, create an
update_ref_oid which wraps update_ref and provides this functionality.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/refs.h b/refs.h
index 1b02043..7a77f3e 100644
--- a/refs.h
+++ b/refs.h
@@ -477,6 +477,9 @@ void ref_transaction_free(struct ref_transaction *transaction);
 int update_ref(const char *msg, const char *refname,
 	       const unsigned char *new_sha1, const unsigned char *old_sha1,
 	       unsigned int flags, enum action_on_err onerr);
+int update_ref_oid(const char *msg, const char *refname,
+	       const struct object_id *new_oid, const struct object_id *old_oid,
+	       unsigned int flags, enum action_on_err onerr);
 
 int parse_hide_refs_config(const char *var, const char *value, const char *);