tests/test-check-rust-format.t

issue6528: implement _is_revision_affected_fast using callback

The delta comming from a bundle/stream does not exists in the revlog yet, so we
will need other way to retrieve the same information.

To prepare for this we split the function to use callbacks in the core logic.

Differential Revision: https://phab.mercurial-scm.org/D11268

#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