Hook up `git refs verify` in housekeeping

Git has grown a new git refs verify tool upstream that allows us to check for whether the reference database is consistent. While the tool right now only checks consistency of the files backend, we'll soonish add support for the reftable backend, as well.

We may want to start using this command as part of housekeeping when we repack references. This would mostly be done as a safety mechanism in the context of the rollout of the reftable backend to production so that we can get alerts as soon as we detect any kind of corruption with the new backend.