The assistant should help the user recover their repository when things go
wrong.

## dangling lock files

There are a few ways a git repository can get broken that are easily fixed.
One is left over index.lck files. When a commit to a repository fails,
check that nothing else is using it, fix the problem, and redo the commit.

This should be done on both the current repository and any local
repositories. Maybe also make git-annex-shell be able to do it remotely?

## incremental fsck

Add webapp UI to enable incremental fsck, and choose when to start and how
long to run each day.

When fsck finds a damanged file, queue a download of the file from a
remote. If no accessible remote has the file, prompt the user to eg, connect
a drive containing it.

## git-annex-shell remote fsck

Would be nice; otherwise remote fsck is too expensive (downloads
everything) to have the assistant do. (remote fsck --fast might be worth
having the assistant do)

## git fsck

Have the sanity checker run git fsck periodically (it's fairly inexpensive,
but still not too often, and should be ioniced and niced). 

If committing to the repository fails, after resolving any dangling lock
files (see above), it should git fsck.

If git fsck finds problems, launch git repository repair.

## git repository repair

There are several ways git repositories can get damanged. 

The most common is empty files in .git/annex/objects and commits that refer
to those objects. When the objects have not yet been pushed anywhere.
I've several times recovered from this manually by
removing the bad files and resetting to before the commits that referred to
them. Then re-staging any divergence in the working tree. This could
perhaps be automated.

As long as the git repository has at least one remote, another method is to
clone the remote, sync from all other remotes, move over .git/config and
.git/annex/objects, and tar up the old broken git repo and `git annex add`
it. This should be automatable and get the user back on their feet. User
could just click a button and have this be done.