.arcconfig

context: extract partial nodeid lookup method to scmutil

We will add another caller soon, and there's a non-obvious reason to
use the unfiltered repo that we don't want to copy across the code
base.

Differential Revision: https://phab.mercurial-scm.org/D3189

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