bug#64391: buffer narrowing slowdown regression in emacs 29

[Eli Zaretskii]

> From: Mattias Engdegård <mattias.engdegard@gmail.com>
> Date: Sat, 1 Jul 2023 13:37:08 +0200
> Cc: 64391@debbugs.gnu.org,
>  Gregory Heytings <gregory@heytings.org>,
>  Eli Zaretskii <eliz@gnu.org>
> 
> 1 juli 2023 kl. 06.59 skrev Andrew Cohen <acohen@ust.hk>:
> 
> > Just for fun I simply commented it out (since the test code doesn't use
> > labeled restrictions) and ran the test code, and the problem disappears:
> 
> The attached patch combines narrow-to-region and internal--label-restriction 
> into a single function, internal--narrow-to-region. (We could also add the 
> label as an optional argument to narrow-to-region.)

It does more than that, so I'd appreciate a more detailed description
of the changes and their rationale.

Thanks.