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

[Gregory Heytings]

> And I've found a serious sink of CPU cycles under truncate-lines, and 
> installed a fix on the feature branch.  Gerd, if you have time to 
> eyeball the fix and comment on it, I'd appreciate.  It's commit 350e97d 
> on the branch.  (I can post a more detailed explanation of what I did 
> and why, if that would help, because the code and the functions it calls 
> are somewhat tricky.)

Hmmm...  After 350e97d78e, Isearch locks Emacs with toggle-truncate-lines. 
Recipe:

C-x C-f long-line.xml
C-x x t
C-s </

You have to kill Emacs, C-g does not work.