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.