mq: stabilize update after strip of parent revision
Strip will (if it updates) update to the parent of revs[0], where revs are the
roots of the tree that is stripped.
When revs was list(set) it was thus undefined which root parent it would update
to. With sorted(set) it is at least stable what it updates to. (But it is very
possible that another more useful and predictable behaviour could be defined
... such as updating to the tip-most surviving wd ancestor.)