Repository copy safety check

[andrewchambers]

When a user copies a repository with rsync or cp they do not also update the gc generation and repository id, a user could accidentally cause data loss by copying repositories back and forth without understanding the nature of gc-generation cache invalidation.

We could perhaps provide proactive safety mitigations for such situations, what they would be is still uncertain. In general a user would be better off using 'bupstash sync' to completely eliminate these risks.

[andrewchambers]

I think the fix is going to be invalidate caches if the repository inode changes.