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

[Gregory Heytings]

> With font-lock, it seems to get progressively slower the farther you get 
> along the current (long line).
>
> E.g. you can have a long line spanning several screenfuls without line 
> breaks. When the window is scrolled to the beginning, redisplay is 
> relatively fast (I can press up/down arrows, and they seem responsive).
>
> But if I scroll the window to the end of said long line, up/down 
> commands become much less responsive.
>
> Tested that with today's master and js-mode visiting a minified JS file.

You should have tried the feature/long-lines-and-font-locking branch 
instead, where that problem has been fixed a few hours ago.