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

[Gregory Heytings]

> Like, we're going from a 255 (?) second delay to 2 second delay already 
> without breaking fontification. And yet you're eager to go from 2 
> seconds down to ~0 and sacrifice highlighting correctness?

Yes.  Because as I told you your 2 seconds are 4 for me and 8 for Eli and 
20 for someone else.  And that's in a relatively small file.

Note that if it were 2/4/8/20 seconds once, and then no further slowdowns 
while editing the file, that would perhaps be okay.  But that's not the 
case, you will regularly see a similar 2/4/8/20 seconds delay.