Add basic VC push support.

Add basic VC push support.

* lisp/vc/vc.el (vc-push): New autoloaded command.
* lisp/vc/vc-hooks.el (vc-prefix-map, vc-menu-map): Add vc-push.
* lisp/vc/vc-bzr.el (vc-bzr--pushpull): New, factored from vc-bzr-pull.
(vc-bzr-pull): Reimplement using vc-bzr--pushpull.
(vc-bzr-push): New.
* lisp/vc/vc-git.el (vc-git--pushpull): New, factored from vc-git-pull.
(vc-git-pull): Reimplement using vc-git--pushpull.
(vc-git-push): New.
* lisp/vc/vc-hg.el (vc-hg--pushpull): New, factored from vc-hg-pull.
(vc-hg-pull, vc-hg-push): Reimplement using vc-hg--pushpull.
* doc/emacs/maintaining.texi (Pulling / Pushing):
Rename from "VC Pull".  Mention pushing.
(VC With A Merging VCS, VC Change Log): Update xrefs.
(Branches): Update menu.
* doc/emacs/emacs.texi: Update menu.
* etc/NEWS: Mention this.