.arcconfig

bundlespec: add support for some variants

This way the stream v2 bundle spec can disable the changegroup part while
enabling the stream v2 part.

Differential Revision: https://phab.mercurial-scm.org/D1955

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