In prog-modes/diff-mode.el
;; XEmacs change: no autoload for this for the transition period
(defadvice vc-backend-diff (around diff-mode-vc activate)
;; BEWARE!! when this is autoloaded, CL might not be available
(condition-case ()
(with-current-buffer "*vc-diff*" (setq buffer-read-only nil))
(error nil))
ad-do-it
(condition-case ()
(with-current-buffer "*vc-diff*"
(if (memq major-mode '(fundamental-mode diff-mode)) (diff-mode)))
(error nil)))
The above breaks when the default-major-mode is something other
than fundamental-mode. As when:
(setq default-major-mode 'text-mode)
Please consider changing this to:
...
(if (or (eq major-mode default-major-mode)
(memq major-mode '(fundamental-mode diff-mode)))
(diff-mode))
-jeff