bug#64391: buffer narrowing slowdown regression in emacs 29

[Eli Zaretskii]

> From: Mattias Engdegård <mattias.engdegard@gmail.com>
> Date: Fri, 7 Jul 2023 11:30:11 +0200
> Cc: Eli Zaretskii <eliz@gnu.org>,
>  acohen@ust.hk,
>  64391@debbugs.gnu.org,
>  monnier@iro.umontreal.ca
> 
> Gregory's first two patches and mine are indeed equivalent up to refactoring; 
> either would do. The moving parts are the same, so performance shouldn't 
> differ noticeably. Neither appears to tie our hands for future changes as the 
> interfaces are all internal.

I was considering applying only the first of Gregory's patches,
without the second.  WDYT?