bug#56393: Actually fix the long lines display bug

[Gregory Heytings]

> > Thanks, but couldn't we use the existing BUF_CHARS_MODIFF for that?
>
> I'll have a look, thanks.

And the answer is no.  (I tried to use it, but did not remember the reason 
why it was not usable in this context while writing my previous reply.) 
I don't know what BUF_CHARS_MODIFF records, but it is for example == 2 
before and after C-x i dictionary.json RET.