tests/test-check-rust-format.t

scmutil: extract function that builds revset expr to select bookmark branch

This is needed to process "log -B" option properly. "log" options have to
be translated to a revset expression, not to an evaluated set.

#require rustfmt test-repo

  $ . "$TESTDIR/helpers-testrepo.sh"

  $ cd "$TESTDIR"/..
  $ RUSTFMT=$(rustup which --toolchain nightly-2020-10-04 rustfmt)
  $ for f in `testrepohg files 'glob:**/*.rs'` ; do
  >   $RUSTFMT --check --edition=2018 --unstable-features --color=never $f
  > done