tests/test-check-rust-format.t

errors: set detailed exit code to 20 for locking errors

This is per https://www.mercurial-scm.org/wiki/ErrorCategoriesPlan.

Differential Revision: https://phab.mercurial-scm.org/D9242

#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