Add support for git revert

The `git revert` feature is super useful in reversing changes that have gone wrong. I was a little surprised to see no mention of it anywhere in the documentation, PRs, or issues. (Or anywhere in the codebase, really)

As far as I know there's no way to do it with the commands available today. (If I'm missing something obvious, let me know!)

I suspect it might be a bit of a challenge to implement, but it'd be great to have!