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

[Gregory Heytings]

> I don't know the difference between "slow" and "hang".  Gerd says it 
> eventually finishes, so it isn't a "hang" at least on his machine. And 
> my build is unoptimized, so what is "slow" for you probably qualifies as 
> "hang" for me.

I just tried again, and on my machine with an optimized build C-s </ RET 
takes 220 seconds to finish (and cannot be interrupted).  That's what I 
call "hang".  With 350e97d78e, C-s </ RET takes about 1 second.  That's 
what I call "slow" (without truncate-lines C-s </ RET is instantaneous).

> If you disable lazy-highlighting, does the problem go away?

Yes, as I said to Gerd, with (setq isearch-lazy-highlight nil) the problem 
goes away (and C-s </ RET is instantaneous).

> And if you customize lazy-highlight-interval and 
> lazy-highlight-max-at-a-time to reasonable values, doesn't the "hang" 
> becomes much shorter?

With (setq lazy-highlight-interval 5) the problem does not go away.  With 
(setq lazy-highlight-max-at-a-time 10) the problem goes away, C-s </ RET 
takes about 1 second.