rust-index: core impl for find_gca_candidates and find_deepest
This still follows closely the C original and not able to treat more than 63
input revisions (bitset backed by `u64` and one bit reserved for poisoning).
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}