bug#49731: 28.0.50; Filter xref results by filename

[Juri Linkov]

>> 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:

This is pushed now as well.