These are helper plugins that mimic the repo plugin: https://github.com/heroku/heroku-repo
The following commands will remain unimplemented:
- `clone`: not useful for the host client to clone
- `download`: currently not possible for us to "host" a tarball of the repo
- `reset`: we still store files in the repo (though we shouldn't) so we cannot easily reinitialize the repository.
Also fixes a potential race condition if two separate processes create an application's `$CACHE_DIR`.
Closes#1570
