code: function checkFile(path) {
    const formatter = path.match(/\.md$/) ? prettierFormat : clangFormat;
    const source = fs.readFileSync(path, 'utf8');
    const formatted = formatter(path, source);
    if (source !== formatted) {
      return formatted;
    }
    return null;
  }
docs: Checks that a path is formatted.
  @return new text of file if it needs formatting or null otherwise.