bug#13949: 24.4.1; `fill-paragraph' should not always put the buffer as modified

[Drew Adams]

> >> Emacs thinks that the buffer is modified when actually it isn't,
> >
> > Emacs correctly tells you that the buffer has been modified.  That the
> > buffer's contents now match the file's contents is irrelevant.
> 
> It is very relevant for me and, judging by the comments of others on
> this and bug report and its duplicates, it is relevant for them too.
> 
> The tools must be adapted to the mental model of their users.

Yes, and:

1. Different users have different use cases and different mental
   models.

2. The same user has different mental models, depending on the
   current context.

3. Code too, not just users, needs to be able to detect buffer
   changes.

4. There are different kinds of buffer changes.  It would be
   good to have ways to detect these as such.