tests/test-check-format.t

grep: improve test coverage

Differential Revision: https://phab.mercurial-scm.org/D8542

#require black

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