.arcconfig

registrar: move "constant" possiblecmdtypes to class level

While at it, switch to set literal syntax.

Differential Revision: https://phab.mercurial-scm.org/D1187

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