Syncing works! I have two clones, and any file I create in the first
is immediately visible in the second. Delete that file from the second, and
it's immediately removed from the first.

Most of my work today felt like stitching existing limbs onto a pre-existing
monster. Took the committer thread, that waits for changes and commits them,
and refashioned it into a pusher thread, that waits for commits and pushes
them. Took the watcher thread, that watches for files being made,
and refashioned it into a merger thread, that watches for git refs being
updated. Pulled in bits of the `git annex sync` command to reanimate this.

It may be a shambling hulk, but it works.

Actually, it's not much of a shambling hulk; I refactored my code after
copying it. ;)

I think I'm up to 11 threads now in the new
`git annex assistant` command, each with its own job, and each needing
to avoid stepping on the other's toes. I did see one MVar deadlock error
today, which I have not managed to reproduce after some changes. I think
the committer thread was triggering the merger thread, which probably
then waited on the Annex state MVar the committer thread had held.

Anyway, it even pushes to remotes in parallel, and keeps track of remotes
it failed to push to, although as of yet it doesn't do any attempt at
periodically retrying.

One bug I need to deal with is that the push code assumes any change
made to the remote has already been pushed back to it. When it hasn't, 
the push will fail due to not being a fast-forward. I need to make it
detect this case and pull before pushing.

(I've pushed this work out in a new `assistant branch`.)