bug#49731: 28.0.50; Filter xref results by filename

[Juri Linkov <juri@linkov.net>]

> Also there is an additional helper function that is needed
> to keep hidden outlines and restore them after reverting the
> xref buffer with 'g' (xref-revert-buffer).
> This is an example of advice that does this.
> Later when xref will use revert-buffer-function,
> this advice could be replaced by a simple hook call:
>
> #+begin_src emacs-lisp
> (define-advice xref-revert-buffer (:around (ofun &rest args) keep-outlines)
>   "Keep hidden outlines after xref revert."
>   (let ((regexp (outline-hidden-headings-regexp))
>         (value (apply ofun args)))
>     (outline-hide-by-heading-regexp regexp)
>     value))
> #+end_src

So here is the patch for xref:

diff --git a/lisp/progmodes/xref.el b/lisp/progmodes/xref.el
index 0025f1f9479..0a5c9243027 100644
--- a/lisp/progmodes/xref.el
+++ b/lisp/progmodes/xref.el
@@ -1277,13 +1277,18 @@ xref-revert-buffer
   "Refresh the search results in the current buffer."
   (interactive)
   (let ((inhibit-read-only t)
-        (buffer-undo-list t))
+        (buffer-undo-list t)
+        restore-functions)
+    (when (boundp 'revert-buffer-restore-functions)
+      (run-hook-wrapped 'revert-buffer-restore-functions
+                        (lambda (f) (push (funcall f) restore-functions) nil)))
     (save-excursion
       (condition-case err
           (let ((alist (xref--analyze (funcall xref--fetcher)))
                 (inhibit-modification-hooks t))
             (erase-buffer)
-            (xref--insert-xrefs alist))
+            (prog1 (xref--insert-xrefs alist)
+              (mapc #'funcall (delq nil restore-functions))))
         (user-error
          (erase-buffer)
          (insert