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

[Juri Linkov <juri@linkov.net>]

>> Perhaps we could just change the .+ thing to .* and see whether it
>> breaks anything?
>
> Good point.
>
> But am I missing something or commit f0768d3145 didn't change
> `diff-hunk-header-re' matching an entire line? I thus don't understand
> why Juri is seing a regression w.r. to cycling after that commit has
> been installed. Note that I've not been able to reproduce the "working"
> behavior described by Juri after reverting that commit.
>
> Juri can you confirm there's a relation between cycling not working and
> that commit? Otherwise should we open another bug?

I confirm there is a relation because the commit f0768d3145

  +  (when (eq diff-buffer-type 'git)
  +    (setq diff-outline-regexp
  +          (concat "\\(^diff --git.*\n\\|" diff-hunk-header-re "\\)"))
  +    (setq-local outline-level #'diff--outline-level))
  +  (setq-local outline-regexp diff-outline-regexp))

changed diff-outline-regexp to contain diff-hunk-header-re that matches an entire line.
This commit replaced the default value of diff-outline-regexp that is

  (defvar diff-outline-regexp
    "\\([*+][*+][*+] [^0-9]\\|@@ ...\\|\\*\\*\\* [0-9].\\|--- [0-9]..\\)")

that didn't match entire lines.

However, changing the .+ thing to .* in outline-font-lock-keywords
fixes this problem.  So we need to decide whether to try to remove
entire line mathing from diff-hunk-header-re (not sure how easy to do),
or just change .+ to .* in outline-font-lock-keywords.