bug#51016: 28.0.50; 'diff-font-lock-prettify' breaks display of outline headers

[Juri Linkov]

>>> which argues for your second suggestion, changing
>>> `outline-font-lock-keywords'.
>>
>> Yup.  Juri, please go ahead and make the change -- it seems like the
>> correct fix ere.
>
> So now pushed to master.

This change has such effect that now page delimiters are highlighted
as well.  For an unknown reason, the default value of outline-regexp
includes the page delimiter ^L: "[*^L]+".  So now outline-font-lock-keywords
fontifies them too with the `outline-1' face.  But this highlighting
is not visible, because by default it's overridden by the hardcoded face
`escape-glyph' on ^L.

This begs the question: why outline-regexp includes ‘^L’?
With it the collapsed outlines take twice more space with empty lines,
and without it the outline overview is more compact: