bug#56393: Actually fix the long lines display bug

[Gregory Heytings]

> > That's wrong, sorry.  Try opening hugedictionary.json in these editors 
> > and you'll see the effect.
>
> I was thinking of much smaller files that Emacs chokes on but e.g. vim 
> does not.

Surely, different editors have different limits.  My aim was to set the 
limit as high as possible for Emacs.  As far as I can tell the only limit 
is now RAM.  I just tried:

for I in $(seq 1 600); do cat dictionary.json; done > monsterdictionary.json

which produces a file with a single 11GB (!) long line, and you can edit 
it almost as it it were a 1KB file with Emacs.

That being said, my main point here was that all editors disable 
non-essential features when they open "big" files, for some definition of 
"big".  Both VS Code and Atom (silently) disable most highlighting even 
for the 300KB long-line.xml file (only "short" lines are highlighted), and 
(silently) disable all highlighting for the 19MB dictionary.json file.