.arcconfig

blackbox: don't unpack the list while passing into str.join()

The current state may result in error TypeError.
Caught using evolve-tests.

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