.arcconfig

phases: define an official tuple of mutable phases

Such tuple was already manually defined in a couple of place. Having an
official definition makes it easy to introduce of new phases.

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