git-annex has good support for running commands in parallel, but there
are still some things that could be improved, tracked here:

* Maybe support -Jn in more commands. Just needs changing a few lines of code
  and testing each.

* Maybe extend --jobs/annex.jobs for more control. `--jobs=cpus` is already
  supported; it might be good to have `--jobs=cpus-1` to leave a spare
  cpu to avoid contention, or `--jobs=remotes*2` to run 2 jobs per remote.
