Resurrect the ability to specify a revision in vc-next-action

Resurrect the ability to specify a revision in vc-next-action

* lisp/vc/vc-bzr.el (vc-bzr-checkin):
* lisp/vc/vc-dav.el (vc-dav-checkin):
* lisp/vc/vc-git.el (vc-git-checkin):
* lisp/vc/vc-hg.el (vc-hg-checkin):
* lisp/vc/vc-mtn.el (vc-mtn-checkin): Accept and silently ignore
an additional optional argument, the revision to checkin.

* lisp/vc/vc-sccs.el (vc-sccs-checkin):
* lisp/vc/vc-cvs.el (vc-cvs-checkin):
* lisp/vc/vc-rcs.el (vc-rcs-checkin): Allow to optionally specify
a revision to checkin.

* lisp/vc/vc.el (vc-next-action): Allow to optionally specify the
revision when checking in files.

See http://lists.gnu.org/archive/html/emacs-devel/2015-09/msg00688.html
for the details.