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 I found a scenario in which redisplay is still very slow, as slow as 
> the master branch.  Here, try this:
>
> emacs -Q
> C-x C-f long-line.xml RET
>
> Now, do NOT disable font-lock, and wait for Emacs to say "Valid" in the 
> mode line (to get nXML mode out of the way).  Then:
>
> M-x toggle-truncate-lines RET

BTW, there is now a C-x x t binding for that.

> Now simple cursor motion commands that use redisplay optimizations are 
> fast, but commands that cause more thorough redisplay are as slow as on 
> master.  As a simple example, try just "M-x" and wait until the "M-x" 
> prompt appears in the minibuffer -- here it takes much longer, basically 
> as long as the version on master.

Yes, it's a font locking issue.  Turn font-lock-mode off and the problem 
is gone.  As I said, I'll look at that later.