Seeing a diff when writing a commit in magit

gistfile1.el

;; look at diff when writing a commit message
(defun magit-log-show-diff ()
  (interactive)
  (let ((content (magit-cmd-output "git" '("diff" "--cached" "-U5"))))
    (with-current-buffer (get-buffer-create "*vc-diff*")
      (let ((buffer-undo-list t)
            (inhibit-read-only t))
        (erase-buffer)
        (insert content))
      (diff-mode)
      (setq buffer-read-only t)
      (display-buffer (current-buffer)))))

(define-key magit-log-edit-mode-map (kbd "C-c C-d") 'magit-log-show-diff)