Create a better mechanism for counting revisions

RevisionTreeIndex.php has this comment:

  /**
   * {@inheritdoc}
   *
   * @todo The revision tree also contain missing revisions. We need a better
   * way to count.
   */
  public function countRevs($uuid) {
    $values = $this->buildTree($uuid);
    return count($values['default_branch']);
  }