gistfile1.el

(defun timvisher/ack-from-root (arg regexp)
  (interactive "P")
  (let ((current-prefix-arg (cond ((not arg) 4)

                                  ((= 4 (car arg)) 16)

                                  ((<= 16 (car arg)) nil))))
    (call-interactively 'ack)))

(defun timvisher/isearch-ack-from-root (arg regexp)
  (interactive
   (list current-prefix-arg
         (cond
          ((functionp isearch-word)
           (funcall isearch-word isearch-string))
          (isearch-word (word-search-regexp isearch-string))
          (isearch-regexp isearch-string)
          (t (regexp-quote isearch-string)))))
  (let ((ack-command (concat "ag " regexp))
        (current-prefix-arg 4))
    (isearch-done nil)
    (isearch-clean-overlays)
    (call-interactively 'ack)))

(define-key isearch-mode-map (kbd "C-c r g") 'timvisher/isearch-ack-from-root)