Finally got the assistant to repair git repositories on removable drives,
or other local repos. Mostly this happens entirely automatically, whatever
data in the git repo on the drive has been corrupted can just be copied
to it from `~/annex/.git`.

And, the assistant will launch a git fsck of such a repo whenever it fails
to sync with it, so the user does not even need to schedule periodic fscks.
Although it's still a good idea, since some git repository problems don't
prevent syncing from happening.

Watching git annex heal problems like this is quite cool!

One thing I had to defer till later is repairing corrupted gcrypt
repositories. I don't see a way to do it without deleting all the objects
in the gcrypt repository, and re-pushing everything. And even doing that
is tricky, since the `gcrypt-id` needs to stay the same.
