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>

refs.c

@@ -858,6 +858,14 @@ int ref_transaction_verify(struct ref_transaction *transaction,
 				      flags, NULL, err);
 }
 
+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)
+{
+	return update_ref(msg, refname, new_oid ? new_oid->hash : NULL,
+		old_oid ? old_oid->hash : NULL, flags, onerr);
+}
+
 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)

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 *);