bisect: merge ancestor lists when pushing to children - eliminate some redundant tests and assignments - move ancestor list merging to child update

bisect: skip calculations on candidates with too many ancestors Once an ancestor list has grown past the perfect threshold, all descendants are less optimal. Use a poison dict to avoid pointless operations on their long ancestor lists, thus eliminating most of the work.

bisect: stop early if we find a perfect candidate

bisect: find best node in ancestor collection pass

bisect: calculate candidate set while finding children

bisect: use a dict for children We fill in the children only for ancestors of badrev