bug#56393: Actually fix the long lines display bug

[Gregory Heytings]

> > Then here's another recipe with long lines detection enabled:
> >
> > emacs -Q
> > C-x C-f src/xdisp.c RET
> > C-n
> > C-x i .../dictionary.json RET
> > M->
> > C-u 37100 C-p
>
> Where can I get a one-line dictionary.json file?  I don't think I have 
> it here.

It's here: 
https://github.com/Wilfred/ReVo-utilities/blob/a4bdc40dd2656c496defc461fc19c403c8306d9f/revo-export/dictionary.json?raw=true

Then you can obtain the 1GB file with:

for I in $(seq 1 60); do cat dictionary.json; done > hugedictionary.json

(Note that I did not actually try the above recipe, it's just meant to 
show that there will always be corner cases that won't be handled, even 
with a dynamic detection of long lines.)