.arcconfig

fix: warn when a fixer doesn't have a configured command

It seems we currently produce an empty command line and then decide to
not run it, but it seems better to skip that part too.

Differential Revision: https://phab.mercurial-scm.org/D7085

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