function unifiedDiff (err) {
  var indent = '      ';
  function cleanUp (line) {
    if (line[0] === '+') {
      return indent + colorLines('diff added', line);
    }
    if (line[0] === '-') {
      return indent + colorLines('diff removed', line);
    }
    if (line.match(/@@/)) {
      return '--';
    }
    if (line.match(/\\ No newline/)) {
      return null;
    }
    return indent + line;
  }
  function notBlank (line) {
    return typeof line !== 'undefined' && line !== null;
  }
  var msg = diff.createPatch('string', err.actual, err.expected);
  var lines = msg.split('\n').splice(5);
  return '\n      ' +
    colorLines('diff added', '+ expected') + ' ' +
    colorLines('diff removed', '- actual') +
    '\n\n' +
    lines.map(cleanUp).filter(notBlank).join('\n');
}
Returns a unified diff between two strings.

@api private
@param {Error} err with actual/expected
@return {string} The diff.