bug#56393: Actually fix the long lines display bug

[Gregory Heytings]

> > I don't think we can detect long lines reliably enough.  The problem of 
> > the original implementation is what Gerd mentioned: "What happens when 
> > evaluating an expression in *scratch* that returns a really large 
> > result?
>
> That problem doesn't exist when you run the detection code at the 
> beginning of redisplay (either in redisplay_window or in start_display 
> or in init_iterator), because by the time redisplay runs the buffer text 
> was already updated by the insertion that is the result of the 
> evaluation.

Wouldn't running such detection code at the beginning of each redisplay be 
too expensive?

> > Note that we the current implementation does not always restrict 
> > display code from seeing the entire buffer, it does so in a few 
> > well-chosen places, everywhere else the display code sees the entire 
> > buffer.
>
> And this is enough to make Emacs responsive?  If yes, that's great.

It is.  On master C-p at the end of dictionary.json loaded "literally" 
takes 27 seconds.  Now the effect is immediate.  On master inserting a 
character there takes several (about 5) seconds, now the effect is 
immediate.  On master you cannot even move to the end of 
hugedictionary.json (the 1 GB file) for example with C-e (or perhaps you 
can, but I gave up after a few minutes).  Now you can move there 
instantly, and the effect of commands there is immediate, too.