discovery: use a lower level but faster way to retrieve parents
We already know that no revision in the undecided set are filtered, so we can
skip multiple checks and directly access lower level data.
In a private pathological case, this improves the timing from about 70 seconds
to about 50 seconds. There are other actions to be taken to improve that case,
however this gives an idea of the general overhead.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}