Previously, posting a new review for a non head commit would orphan the head.
The general case is any descendant of the selected revisions got orphaned if
this was the first time the selected revisions were submitted. It doesn't
happen when resubmitting. I've already had coworkers hit this a few times and
get confused. Since posting a review isn't generally thought of as an editing
operation, it would probably be easier for new users if we just restacked.
This avoids restacking existing orphans around the submission because that may
involve merge conflict resolution. Users who already have orphans should know
how to stabilize them anyway.