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

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Eli Zaretskii <eliz@gnu.org>,  13949@debbugs.gnu.org
> Date: Thu, 14 Mar 2013 09:38:08 -0400
> 
> > 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.

This has the disadvantage of scanning the entire buffer, which might
increase paging and memory pressure in general.

> - 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.

But it sounds like Dani wants this behavior not only for
fill-paragraph, but for any command that can potentially modify the
buffer, but actually doesn't.  This would require to compute sha1
before and after every command that might change the buffer.