bug#56393: Actually fix the long lines display bug

[Gregory Heytings]

Thanks for your feedback!

> There's some slight hangs when `C-s'-ing the file, but none that last 
> longer than a second or two.  (Perhaps that has something to do with 
> `C-s' trying to add highlights to all the matches?)

Indeed.  I want to preserve normal editing experience as far as possible, 
so I did not touch these values so far.  I tried the four possible t/nil 
combinations for search-highlight and isearch-lazy-highlight, and setting 
isearch-lazy-highlight to nil improves Emacs' response time in such cases. 
Also setting isearch-highlight to nil is better, but only slightly.  I'll 
see what I can do.