message: Use shutil.which() instead of shelling out
  Summary: Supported since Python 3.3. Shelling out to /bin/which also produced annoying stderr spew when the binary wasn't found.
diff: @@ -143,12 +143,8 @@ def get_stop_pass_idx(passes_list, pass_name_and_num):
  def run_redex_binary(state):
  if state.args.redex_binary is None:
  - try:
  - state.args.redex_binary = (
  - subprocess.check_output(["which", "redex-all"]).rstrip().decode("ascii")
  - )
  - except subprocess.CalledProcessError:
  - pass
  + state.args.redex_binary = shutil.which("redex-all")
  +
  if state.args.redex_binary is None:
  # __file__ can be /path/fb-redex.pex/redex.pyc
  dir_name = dirname(abspath(__file__))
  