org-src.el: Make sure `buffer-file-name' is always nil.

* org-src.el (org-edit-src-code): Make sure `buffer-file-name'
is always nil.

Thanks to Leo Alekseyev in hunting this down.

lisp/org-src.el

@@ -324,6 +324,7 @@ buffer."
 	 (if org-src-preserve-indentation col (max 0 (- col total-nindent))))
 	(org-src-mode)
 	(set-buffer-modified-p nil)
+	(setq buffer-file-name nil)
 	(and org-edit-src-persistent-message
 	     (org-set-local 'header-line-format msg))
 	(let ((edit-prep-func (intern (concat "org-babel-edit-prep:" lang))))