No, wait. The use case here is what to call the refs that are backed up. The default behavior is to copy the rewritten refs to `refs/original` (i.e. the old `refs/heads/master` is copied to `refs/original/heads/master` before the new `refs/heads/master` is written). Returning null means that you don't want to keep a backup.

I'm going to rewrite this as I mentioned above, I think it provides more power to the caller. Stay tuned.