discovery: simply walk the undecided revs when building the children mapping The sampling only care about revisions in the undecided set, so building children relationship within this set is sufficient. The set of undecided changesets can be much smaller than the full span from its smallest item to the tip of the repository. This restriction can significantly speed up operations in some cases. For example, on our private pathological case, this speeds things up from about 53 seconds to about 7.5 seconds.