D3728: grep: adds unmodified mode

[Yuya Nishihara]

> > commands.py:2411
> >       _('only search files changed within revision range'), _('REV')),
> > +    ('', 'unmodified', False,
> > +     _('include all files in the changeset while grepping')),
> 
> I wonder if `--allfiles` is a better name.  `--unmodified` makes me think unmodified files exclusively.
> 
> It's too bad `--all` is already used, because that would be consistent with the all files meaning in `status` and `revert`.  (Maybe others?)  The plan page mentions BCing plain grep into oblivion, and deprecating `--all`.  Is it worth a BC on this too, for consistency going forward?

Maybe we can keep all new flags hidden as "(EXPERIMENTAL)" until we make
the final BC? I don't think it's time for bikeshedding yet.