[[done]]

I've been considering adding a `git-annex-shell` command. This would
be similar to `git-shell` (and in fact would pass unknown commands off to
`git-shell`).

## Reasons

* Allows locking down an account to only be able to use git-annex (and
  git).
* Avoids needing to construct complex shell commands to run on the remote
  system. (Mostly already avoided by the plumbing level commands.)
* Could possibly allow multiple things to be done with one ssh connection
  in future.
* Allows expanding `~` and `~user` in repopath on the remote system.
