Repeat undo-only is not working

[Ergus <spacibba@aol.com>]

Hi Juri:

I have this config:

```
(global-set-key (kbd "C-/") #'undo-only)
(global-set-key (kbd "C-M-/") #'undo-redo)

(with-eval-after-load 'repeat
   (defvar undo-redo-repeat-map
     (let ((map (make-sparse-keymap)))
       (define-key map "/" #'undo-only)
       (define-key map "M-/" #'undo-redo)
       (define-key map "U" #'undo)
       map)
     "Keymap to repeat undo-redo key sequences.  Used in `repeat-mode'.")
   (put 'undo-only 'repeat-map 'undo-redo-repeat-map)
   (put 'undo-redo 'repeat-map 'undo-redo-repeat-map)
   (put 'undo 'repeat-map 'undo-redo-repeat-map))
```

And for some reason the repeat map does not activates with "undo-only"
(it doesn't even shows the message in the minibuffer) but with "undo"
and "undo-redo" it works as expected. Is there anything special in
"undo-only" that disables repeat-mode?