bug#56682: Fix the long lines font locking related slowdowns

[Stefan Monnier]

>>> This is needed by Org parser to determine the syntax element at point.
>> But font-lock widens before using Org-mode's `font-lock-keywords`, so
>> there should be no need for Org-mode to widen in that case.
> Yes, but Org parser is not written explicitly for font-locking purposes.
> org-element-at-point knows nothing about buffer restriction upon
> calling and thus has to widen to ensure correctness.

That will break uses of MMM-mode where one of the chunks is using
Org-mode.  Admittedly, this is unlikely, but still: It would be much
better to arrange to do `widen` *outside* of Org-mode's font-lock (and
indentation) code.

>> PS: Not sure if Org-mode qualifies as a mode that is likely to suffer
>> from the kinds of long-lines or large-file problems that Gregory has
>> tried to tackle, tho.  I've never seen an Org file with lines >10kB nor
>> an Org file larger than 10MB.
>
> I have seen very long lines in Org. Specifically, in source block
> output. (http query)
>
> My notes.org file is over 20Mb.
> Even larger files can be auto-generated by 
> https://gitlab.com/publicvoit/Memacs

I stand corrected,


        Stefan