Migrate objects to the repository's configured object directory

Our quarantine helpers always migrate the objects from the quarantine to '<repo.git>/objects'. This works if the repository hasn't been configured with a different object directory which may be the case if the repository is already quarantined. If that's the case, we should instead migrate the objects to the configured object directory. Support this by considering the configured object directory before defaulting to migrating to 'objects'.

While double quarantines should be avoided, they allow for migrating all existing RPCs to a new quarantining approach without having to touch all of the RPCs in the initial migration.