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
#require rustfmt test-repo
$ . "$TESTDIR/helpers-testrepo.sh"
$ cd "$TESTDIR"/..
$ RUSTFMT=$(rustup which --toolchain nightly rustfmt)
$ for f in `testrepohg files 'glob:**/*.rs'` ; do
> $RUSTFMT --check --edition=2018 --unstable-features --color=never $f
> done