New global minor mode vc-auto-revert-mode

New global minor mode vc-auto-revert-mode

* lisp/vc/vc-hooks.el (auto-revert-mode): Declare.
(vc-auto-revert-mode): New global minor mode.
(vc-turn-on-auto-revert-mode-for-tracked-files): New function.
* lisp/vc/vc-dispatcher.el (auto-revert-mode)
(auto-revert-buffers): Declare.
(vc-resynch-window): Don't call vc-revert-buffer-internal when
auto-revert-mode will revert the buffer.  Call
auto-revert-buffers to ensure that this reversion happens in a
timely manner.
* lisp/vc/vc.el (vc-register): Apply vc-auto-revert-mode to
buffers visiting newly registered files.
* lisp/emacs-lisp/easy-mmode.el (define-globalized-minor-mode):
Improve the generated docstring.
* doc/emacs/vc1-xtra.texi (VC Auto-Reverting):
* etc/NEWS: Document the new minor mode.

(cherry picked from commit 9d750c7e8041437758c919f6088d6f3686847812)