bug#56682: Fix the long lines font locking related slowdowns

[Dmitry Gutov]

On 02.08.2022 10:55, Stefan Monnier wrote:
> > only top and bottom of the buffer are fontified), with the caveat that
> > font-lock always tries to backtrack to BOL when fontifying the current
> > hunk. Which makes sense, of course, but could be tweaked for long lines to
> > avoid re-fontifying the whole buffer again and again.
> It has already been (recently) tweaked for this on `master`.

Sorry I missed that. Commit 15b2138719b34083967001c3903e7560d5e0947c, right?

Then, as long as redisplay avoids narrowing when running 
fontification-functions, everything should work well.

Or at least much better than before.