Geo mark unclonable repositories

In gitlab-com/infrastructure#3838 we discovered a number of repositories failed to clone on the Geo secondary.

In some way we should be able to do some kind of check (git fsck?) on the primary and mark those repos as faulty and avoid Geo to sync them.

If it seems they are completely invalid, we have to find a way to recover the faulty data, or worst case, delete the repo if that's not possible.

Related issue

Part of gitlab-org/gitlab-ee#4746