Support "reverting" Xref buffers (bug#35702)

Support "reverting" Xref buffers (bug#35702)

* lisp/progmodes/xref.el (xref--fetcher): New variable.
(xref--xref-buffer-mode-map): Add binding for 'g'.
(xref--revert-xref-buffer): New command.
(xref--show-xref-buffer): Accept a function as the first argument.
(xref--show-xrefs): Same.
(xref--find-xrefs): Pass the above a fetcher function.

* lisp/progmodes/project.el (project-find-regexp)
(project-or-external-find-regexp): Same.

* lisp/dired-aux.el (dired-do-find-regexp): Same.