Initial support for unified DVCS pull and merge.

Initial support for unified DVCS pull and merge.

* lisp/vc/vc-bzr.el (vc-bzr-admin-branchconf, vc-bzr-history): New vars.
(vc-bzr--branch-conf, vc-bzr-async-command, vc-bzr-pull)
(vc-bzr-merge-branch): New functions, implementing merge-branch
and pull operations.

* lisp/vc/vc.el (vc-merge): Use vc-BACKEND-merge-branch if available.
Accept optional prefix arg meaning to prompt for a command.
(vc-update): Use vc-BACKEND-pull if available.  Accept optional
prefix arg meaning to prompt for a command.
(vc-pull): Alias for vc-update.