function unifiedDiff(err, escape) {
  var indent = '      ';
  function cleanUp(line) {
    if (escape) {
      line = escapeInvisibles(line);
    }
    if (line[0] === '+') {
      return indent + colorLines('diff added', line);
    }
    if (line[0] === '-') {
      return indent + colorLines('diff removed', line);
    }
    if (line.match(/\@\@/)) {
      return null;
    }
    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(4);
  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
@param {boolean} escape
@return {string} The diff.