.arcconfig

upgrade: start moving the "to be happening" data in a dedicated object

The upgrade code has a lot of logic to determine which action needs to be
performed depending of various element (sometimes depending from each other). It
would be nice to have a consistent object representing this. That could be
cleanly passed and avoid some logic duplication.

So we create this object as a start.

Differential Revision: https://phab.mercurial-scm.org/D9487

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