.arcconfig

rust: rustfmt config for hg-direct-ffi

For now, we're duplicating it, but it would be probably a good idea
to use a single one for the whole workspace (would have implications on the
other crates as well)

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