Teach VC-Dir to automatically add and remove marks on other lines

Teach VC-Dir to automatically add and remove marks on other lines

* lisp/vc/vc-dir.el (vc-dir-allow-mass-mark-changes): New option.
(vc-dir-parent-marked-p): Replace with ...
(vc-dir--parent): ... this.
(vc-dir-children-marked-p): Replace with ...
(vc-dir--children): ... this.
(vc-dir-mark-file): Unmark subitems before marking a directory.
Offer to unmark a directory before marking a subitem.
(vc-dir-unmark-file): For an implicitly marked item, offer to
unmark it by marking everything else that's implicitly marked.
For an unmarked directory with marked subitems, offer to unmark
them all.
* etc/NEWS: Document the changes.

(cherry picked from commit 936b2efdb389488d291086d5c2189fd1a7170aa6)