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

[Gregory Heytings]

> emacs -Q
> C-x C-f long-line.xml RET
> M-g g 12
> C-u 12 M-x duplicate-line
> C-x C-s multiple-long-lines.xml RET
> C-x C-f multiple-long-lines.xml RET y
> C-x x t
> C-s </ RET

Sorry, there's a typo in that recipe, it should be:

C-u 50 M-x duplicate-line