With this change I'm leaving in the -b as it now makes sense. The command that is posted as output is the (first) one of the commands that affected that branch and were undone.

This solution doesn't handle the general case.

Current test plan involves doing the following:

1. Running a bash script that sets up an interesting repo
2. hitting hg undo -p and trying every key combination Specifically: control-C still works, q quits, left goes forward, right goes backwards, return brings you to a state
3. running redo after preview and return
4. running undo a few times and then using undo -p to go back forward
5. going out of bounds both up and down
6. hitting other random keys during hg undo -p
7. running hg undo -p with other flags. Note that --branch doesn't give you the correct preview

{if(revset("{rev} and UNDODEL"), "-")} type stuff is on the facebook.rc side, so there isn't much to address about it here. The revsets/templates to do it exist and I've tested it.