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

[Juri Linkov]

>> This begs the question: why outline-regexp includes ‘^L’?
>
> Yes, I wondered about that, too.

I guess no one will notice the absence of ^L because
the default outline-regexp used by everyone in org-mode is
"\\*+ "

diff --git a/lisp/outline.el b/lisp/outline.el
index 5e3d4e0e00..603795cfd3 100644
--- a/lisp/outline.el
+++ b/lisp/outline.el
@@ -42,7 +42,7 @@ outlines
   :prefix "outline-"
   :group 'text)
 
-(defvar outline-regexp "[*\^L]+"
+(defvar outline-regexp "[*]+"
   "Regular expression to match the beginning of a heading.
 Any line whose beginning matches this regexp is considered to start a heading.
 Note that Outline mode only checks this regexp at the start of a line,
--