undo-redo.el

(unless (boundp 'undo--last-change-was-undo-p)
  (defun undo--last-change-was-undo-p (undo-list)
    (while (and (consp undo-list) (eq (car undo-list) nil))
      (setq undo-list (cdr undo-list)))
    (gethash undo-list undo-equiv-table))

  (defun undo-redo (&optional arg)
    "Undo the last ARG undos."
    (interactive "*p")
    (cond
     ((not (undo--last-change-was-undo-p buffer-undo-list))
      (user-error "No undo to undo"))
     (t
      (let* ((ul buffer-undo-list)
             (new-ul
              (let ((undo-in-progress t))
                (while (and (consp ul) (eq (car ul) nil))
                  (setq ul (cdr ul)))
                (primitive-undo arg ul)))
             (new-pul (undo--last-change-was-undo-p new-ul)))
        (message "Redo%s" (if undo-in-region " in region" ""))
        (setq this-command 'undo)
        (setq pending-undo-list new-pul)
        (setq buffer-undo-list new-ul))))))