D4322: setdiscovery: don't use dagutil for parent resolution

[Yuya Nishihara]

> +    # TODO this is quadratic
> +    parentfn = lambda rev: repo.changelog.children(repo.changelog.node(rev))
> +
> +    _updatesample(revs, revsroots, sample, parentfn)

Here `parentfn()` returns a list of nodes, which is wrong. But I took this
since this code is completely rewritten.

Surprisingly, test-setdiscovery.t didn't fail at this revision.