Add autocompletion to the 'from' field

I thought it would be handy if there was an auto completion for the 'from' field. The auto completion would get it's data from a list of 404s in the watchdog table. This data is checked to make sure that the 404s are still not a valid menu path or a current redirect. This added functionality also has a test case written for it just to be sure. Here's the PHP code of the auto complete function that I added:

/**
 * Autocompletion callback for the add/edit redirect form. Returns a list of
 * current 404s on the site.
 */
function path_redirect_autocomplete($string = '') {
  $matches = array();

  // Get a list of 404s, sorted by the number of times each 404 was processed.
  $query = db_query("SELECT message, COUNT(message) AS count FROM {watchdog} WHERE type = 'page not found' AND LOWER(message) LIKE '%%%s%%' GROUP BY message ORDER BY count DESC", drupal_strtolower($string));
  while ($path = db_result($query)) {
    // If the 404 is now a valid path or already has a redirect, discard it.
    if (!menu_get_item($path) && !path_redirect_load(array('path' => $path))) {
      $matches[$path] = check_plain($path);
    }
  }

  // Limit the output to 10 results and return the JSON.
  $matches = array_slice($matches, 0, 10);
  drupal_json($matches);
}