bug#44983: Truncate long lines of grep output

[Juri Linkov <juri@linkov.net>]

>> Maybe instead of using font-lock to hide long parts
>> of grep lines, it would be better to do the same
>> directly in compilation-filter/grep-filter?
>
> I now have a rough patch that does this, but the problem is that even if
> I splat a "..." display over the text, font-lock seems to insist on
> going over the data anyway, so the display is still dog slow.
>
> I thought I remembered there was a way to say to font-lock "ignore this
> bit of the buffer", but I can't find it now.  Do I misremember?

I don't remember such font-lock text property, but now I have no problems
when long lines are hidden initially with:

```
(add-hook 'xref-after-update-hook
          (lambda ()
            (setq-local outline-regexp (if (eq xref-file-name-display 'abs)
                                           "/" "[^ 0-9]")
                        outline-default-state 1
                        outline-default-rules '(subtree-has-long-lines)
                        outline-default-long-line 1000)
            (outline-minor-mode +1)))
```