tests/test-check-format.t

backout: don't assume that tip as what we just committed

Differential Revision: https://phab.mercurial-scm.org/D9256

#require black test-repo

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