bug#15771: 24.3.50; M-q on comment lines marks the buffer modified when it shouldn't

[Jambunathan K]

Here is the recipe.

C-x C-f comment.el RET 
M < 
M q 
C-x C-s 
M q 
M-x diff-buffer-with-file RET C-x C-f comment.el RET 

Here is what the diff produces.  When there are no differences, why is
the buffer marked as modified?

--8<---------------cut here---------------start------------->8---
diff -pub --label /home/kjambunathan/comment.el --label \#\<buffer\ 
comment.el\> /home/kjambunathan/comment.el /tmp/buffer-content-2444dnM

Diff finished (no differences).  Thu Oct 31 22:45:13 2013
--8<---------------cut here---------------end--------------->8---

comment.el
Description: application/emacs-lisp

----------------------------------------------------------------

In GNU Emacs 24.3.50.4 (i686-pc-linux-gnu, GTK+ Version 2.20.1)
 of 2013-10-30 on debian-6.05
Bzr revision: 114868 rgm@gnu.org-20131030102316-8vif7u6ecyo3yieg
Windowing system distributor `The X.Org Foundation', version 11.0.10707000
System Description:     Debian GNU/Linux 6.0.5 (squeeze)