Suggestion: `dorothy revert`

[balupton]

In case an update goes bad, we should offer a revert command to go back to the commit before the update, with confirm for their core and user repos.

this would require saving the commit info into the xdg state dir when running dorothy update

[balupton]

Actually, as the upgrade may have broken everything, having a dorothy revert may not work as it is broken. The best thing to do is to have the update output the prior to new commit, and it already does that. Furthermore I now dev on branches first, make sure everything works, then push to master.