bug#64391: buffer narrowing slowdown regression in emacs 29

[Gregory Heytings]

> > I pushed a proposed change to the scratch/bug64391 branch.
> >
> > It contains three commits:
> >
> > - b741dc7fcd which is the minimal possible commit to fix this bug
> >
> > - 01fb898420 which moves a few statements from the callee 
> > (internal--label-restriction) to its only caller 
> > (internal--labeled-narrow-to-region), and simplifies the code 
> > accordingly
> >
> > - c52ade305e which is optional, and makes the symmetrical change for 
> > the widening case
>
> OK, please merge the first two commits to the release branch, and 
> thanks.

Thanks, now done.

I'll merge the third commit, and the other patch suggested by Mattias, to 
master.