DisconnectGitAlternates: bail out more often

Closes #1660 (closed)

Now that https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28144 is merged, we are going to run DisconnectGitAlternates automatically on a whole lot of repositories. Specifically, it runs during repo housekeeping, whenever the repo is not part of a object deduplication pool.

This change makes it so that if we see unexpected contents in objects/info/alternates, we always just bail out with an error. I think it's better to err on the side of caution.