insert-file-contents disables undo

[Katsumi Yamaoka]

Hi,

When performing `insert-file-contents' at the end of a buffer,
which is not empty, buffer's undo is disabled.  It happens only
with Emacs 23.

(with-temp-buffer
  (buffer-enable-undo)
  (insert "\n")
  (insert-file-contents "~/.emacs")
  buffer-undo-list)
 => t

I was troubled for months with this problem that sometimes
happened when writing a message draft, and I found the cause of
it today.  It happens when I perform `message-insert-signature'
which uses `insert-file-contents' to insert my ~/.signature file.
It is not only on me, is it?  Could someone repair it?

Thanks in advance.