Added two options to git annex fsck that allow for a form of distributed fsck. This is useful in situations where repositiories cannot be trusted to continue to exist, and cannot be checked directly, but you'd still like to keep track of their status. iabackup is one use case for this.

By running a periodic fsck with the --distributed option, the repositories can verify that they still exist and that the information about their contents is still accurate. This is done by doing an extra update of the location log each time a file is verified by fsck to still be in the repository.

The other option looks like --expire="30d somerepo:60d". It checks that each specified repository has recorded a distributed fsck within the specified time period. If not, the repository is dropped from the location tracking log. Of course it can always update that later if it's really still around.

Distributed fsck is not the default because those extra location log updates increase the size of the git-annex branch. I did one thing to keep the size increase small: An identical line is logged to for each key, including the timestamp, so git's delta compression will work as well as is possible. But, there's still commit and tree update overhead.

Probably doesn't make sense to run distributed fscks too often for that and other reasons. If the git-annex branch does get too large, there's always git annex forget ...

(Update: This was later rethought and works much more efficiently now..)