.arcconfig

tests: fix test-check-commit.t when all commits are public

I'm 99% sure this is a portable use of /bin/[, and it seems to fix the
issue I noticed on the buildbot on my machine.

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