tests/test-check-format.t

copies-rust: make the comparison aware of the revision being current merged

This make no significant performance change in practice (all ±2%) in practice,
but it will help us to distinct between some semantically different cases later
on.

Differential Revision: https://phab.mercurial-scm.org/D9498

#require black test-repo

  $ cd $RUNTESTDIR/..
  $ black --config=black.toml --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/**'`