Allow use of vc-root-* commands in *vc-log* buffers.

Allow use of vc-root-* commands in *vc-log* buffers.

* lisp/vc/log-edit.el (log-edit-vc-backend): New variable.
(log-edit): Doc fix.

* lisp/vc/log-view.el (log-view-vc-fileset, log-view-vc-backend): Doc fix.

* lisp/vc/vc-dispatcher.el (vc-log-edit): New args.  Use PARAMS
argument of log-edit to set up all local variables.
(vc-start-logentry): New optional arg specifying VC backend.

* lisp/vc/vc.el (vc-checkin): Use it.
(vc-deduce-fileset): Handle Log Edit buffers.
(vc-diff): Make first argument optional too.