shrink-revlog: add "reverse postorder" and "postorder reverse" toposorts. Based on a patch by Benoit Boissinot, adapted to the pluggable sort algorithm design. toposort_reversepostorder() is a very good performer; it's designed to recreate what the revlog would have looked like if Mercurial had parent deltas now. toposort_postorderreverse() is unstable and very inconsistent, but perhaps with some work it could be made better.