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---