test-check-rust-format: specify --edition=2018
rustfmt doesn't read Cargo.toml unless it's executed by cargo.
https://github.com/rust-lang/rustfmt#rusts-editions
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}