ijump optimization: don't penalize for returning along the parent path

[seveibar]

See how the top movement scores above the left movement? This is (in part) because the left movement is receiving a penalty for traversing too far to the right. We can optimize this by making it so if we travel backwards, we take the parent of our parent. (i.e. the parent of node "1" should be the parent of "X", rather than node "1"'s parent being "X")