bug#13949: 24.3.50; `fill-paragraph' should not always put the buffer as modified

[Stefan Monnier]

> Well, since the `fill-paragraph' command at step #4 leaved the buffer
> with the same contents, flagging the buffer as modified was
> unnecessary in this case.

AFAIK there are two ways to go about it:
- compare the sha1 of the paragraph before and after filling and reset
  buffer-modified-p if it shows the text hasn't changed.
- change fill.el so that filling paragraph doesn't just "unfill whole
  paragraph + fill whole paragraph" but instead goes line by line, and
  only modifies the text where there's a need to.

The second option has the advantage that it truly doesn't modify the
buffer (hence, less font-lock work, less redisplay work, and also
text-properties, overlays and markers aren't affected, contrary to the
current behavior), but it requires more coding effort.


        Stefan