Correctly check buffer mtime when displaying xref matches

Correctly check buffer mtime when displaying xref matches

This was just a typo: we were checking the modification time of
current-buffer instead of checking the modification time of the
passed-in buffer.

This caused matches to not be shown if they weren't present in
the current in-Emacs state of the buffer.

This was easily reproduced by writing a string to a file outside
Emacs, then searching for that string with
e.g. project-find-regexp.  The string would seemingly not be
found, although in reality it was found, just not displayed.

* lisp/progmodes/xref.el (xref--find-file-buffer):
Check buf, not current-buffer (bug#70008).