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

[Gregory Heytings]

> > If it's font-lock that forces syntax-ppss to examine the whole huge 
> > line, then what is your proposal for avoiding that which doesn't 
> > involve some more-or-less arbitrary restrictions on the part of the 
> > buffer that can be examined by syntax-ppss?
>
> The use of `syntax-wholeline-max` in 
> `font-lock-extend-region-wholelines` supposedly fixed this problem since 
> it changed `font-lock` so it doesn't ask `syntax-ppss` to compute the 
> whole line/buffer.

Actually, it didn't, it made things better in some cases, but (much) worse 
in other cases.  You may have seen the recipes I sent to Dmitry a few days 
ago:

emacs -Q
M-: (setq long-line-threshold nil syntax-wholeline-max most-positive-fixnum) RET
C-x C-f dictionary.json RET y ;; takes 160 seconds
C-e ;; takes 200 seconds

emacs -Q
M-: (setq long-line-threshold nil) RET
C-x C-f dictionary.json RET y ;; immediate
C-e ;; not finished after 1200 seconds (20 minutes), I killed Emacs

emacs -Q
C-x C-f dictionary.json RET y ;; immediate
C-e ;; immediate