The Big Refactor: rework 'Local'

[mnuttall]

When reviewing #31 we went down the path of implementing a 'Local' interface which we've subsequently come to regret.

Issues #69 is the main external problem: although we always promote from a Git repository, because we treat local ones as file systems and not git repositories, we can't automatically generate a Pull Request subject of the form, 'Publish service-x at git commit [abc12345] into repo Y' because we can't get a Git commit out of the Local interface.

See https://gist.github.com/bigkevmcd/4ab82d3cce78bceb929b5c364ce91ef7 for some ideas about how the code should be refactored.