src/cli/util.js/listDifferent
function listDifferent(context, input, options, filename) {
  if (!context.argv["list-different"]) {
    return;
  }

  options = Object.assign({}, options, { filepath: filename });

  if (!prettier.check(input, options)) {
    if (!context.argv["write"]) {
      context.logger.log(filename);
    }
    process.exitCode = 1;
  }

  return true;
}