* bug: cannot "git annex ../foo"   (GitRepo.relative is buggy and
  git-ls-files also refuses w/o --full-name, which would need other changes)

* --push/--pull should take a reponame and files, and push those files
  to that repo; dropping them from the current repo

* how to handle git mv file? -> git annex fix -> run automatically?

* how to handle git rm file? (should try to drop keys that have no
  referring file, if it seems safe..)

* add a git annex fsck that finds keys that have no referring file

* Support for remote git repositories (ssh:// specifically can be made to
  work, although the other end probably needs to have git-annex installed..)

* Find a way to copy a file with a progress bar, while still preserving 
  stat. Easiest way might be to use pv and fix up the permissions etc
  after?

* finish BackendChecksum