.arcconfig

debug: add debugwhyunstable that explains instabilities

This is a port of evolve's feature of listing all unstable changesets in detail
(`hg evolve --list`).

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