refs: factor update_ref steps into helpers

refs: factor update_ref steps into helpers

Factor the lock and write steps and error handling into helper functions
update_ref_lock and update_ref_write to allow later use elsewhere.
Expose lock_any_ref_for_update's type_p to update_ref_lock callers.

While at it, drop "static" from the local "lock" variable as it is not
necessary to keep across invocations.

Signed-off-by: Brad King <brad.king@kitware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>