Add a VC command to prepare patches

Add a VC command to prepare patches

* doc/emacs/vc1-xtra.texi (Miscellaneous VC):  Add new node.
(Editing VC Commands):  Document new feature.
* etc/NEWS:  Mention 'vc-prepare-patch'.
* lisp/vc/log-view.el: Autoload 'log-view-get-marked'.
* lisp/vc/vc-git.el (vc-git-prepare-patch):  Add Git implementation.
* lisp/vc/vc-hg.el (vc-git-prepare-patch):  Add Mercurial implementation.
* lisp/vc/vc-bzr.el (vc-git-prepare-patch):  Add Bazaar implementation.
* lisp/vc/vc.el (vc-read-revision):  Add a MULTIPLE argument.
(vc-read-multiple-revisions):  Add an auxiliary function that always
calls 'vc-read-revision' with a non-nil value for MULTIPLE.
(vc-prepare-patches-separately):  Add user option.
(message-goto-body):  Declare function.
(message--name-table):  Declare function.
(vc-default-prepare-patch): Add a default implementation.
(vc-prepare-patch):  Add command.  (Bug#57400)