repository: Consider cruft packs in PruneUnreachableObjects

When pruning objects via PruneUnreachableObjects, then we execute git-prune(1) with a reduced grace period of 30 minutes. This only works to prune loose objects though, and will never delete objects part of a cruft pack. This means that as we're moving unreachable objects into cruft packs, that those objects will in fact not get pruned as expected anymore.

Fix this bug by repacking the repository with cruft packs and a reduced expiry time.