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

[Gregory Heytings]

> I've never seen fontification-functions contain anything but 
> jit-lock-function,

You did not read the thread carefully enough: two weeks ago it was 
mentioned that fontification-functions has at least two other users: in 
ELPA multi-mode.el sets fontification-functions to multi-fontify, and in 
MELPA poly-lock.el sets fontification-functions to poly-lock-function.