/*
* Add a reference update to transaction. new_sha1 is the value that
- * the reference should have after the update, or zeros if it should
+ * the reference should have after the update, or null_sha1 if it should
* be deleted. If have_old is true, then old_sha1 holds the value
* that the reference should have had before the update, or zeros if
* it must not have existed beforehand.