bug#51809: 29.0.50; [PATCH] Support for outline default state in Diff buffers

[Juri Linkov <juri@linkov.net>]

>> I am now back to the origin of this bug report (toggling visibility of
>> some diff hunks depending on long lines, chunks size, or file names) and
>> I see there's a major drawback with the outline mode default visibility
>> state approach: When outline minor mode is set through diff-mode-hook,
>> the buffer is emtpy and applying the default state visibility is a
>> no-op; It's a side-effect of the buffer content being generated
>> asynchronously...
>
> I fixed it using an advice to `vc-diff-finish',

This problem is not specific to diff-mode.  Like I demonstrated
earlier, xref requires using xref-after-update-hook,
so now it could look like this:

#+begin_src emacs-lisp
(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-minor-mode)
            (outline-apply-default-state)))
#+end_src

> but wondering whether there's a clean way to achieve the same...

Some time ago in another bug report I proposed to add a hook
at the end of 'vc-diff-finish', so it will solve this problem.

PS: Please give me more time to test your patch thoroughly
in various modes.