Need an "undo" feature

[goldstar611]

I think we also have an opportunity to provide a better "undo" feature that automatically cleans the index.

We have a contribution for gl revert which is just a wrapper around git revert but something better that handles the index for us would be a step in the right direction.

https://github.com/goldstar611/gitless/pull/36

[sirlucjan]

Like I said, I'd love to test. Tag me and I'll get down to testing.