(defun mwheel-scroll (event)
  (interactive (list last-input-event))
  (let ((button (mwheel-event-button event)))
    (cond ((eq button mouse-wheel-down-event)
           (undo-tree-undo))
          ((eq button mouse-wheel-up-event)
           (undo-tree-redo)))))
           
;; ^ global override for all scrollwheel events
;; to do it only for shift+scroll, try:

(defun mwheel-scroll-through-time (event)
  (interactive (list last-input-event))
  (let ((button (mwheel-event-button event)))
    (cond ((eq button mouse-wheel-down-event)
           (undo-tree-undo))
          ((eq button mouse-wheel-up-event)
           (undo-tree-redo)))))

(global-set-key (kbd "<S-mouse-4>") 'mwheel-scroll-through-time)
(global-set-key (kbd "<S-mouse-5>") 'mwheel-scroll-through-time)