.arcconfig

discovery: explicitly use `undecided` for the children mapping

Recent performance achievements makes the assumption that the `undecided` set
is used for sampling. That assumption is always true in practice. We stop
pretending that taking anything else would make sense and we directly use the
`undecided` set from the object. This provides a more honest API.

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}