Refresh Dired and VC-dir buffers after vc-pull and vc-merge.

Refresh Dired and VC-dir buffers after vc-pull and vc-merge.

* vc/vc-dispatcher.el (vc-set-async-update): New function for
updating Dired or VC-dir buffers after async command completes.

* vc/vc-bzr.el (vc-bzr-async-command): Return the process buffer.
(vc-bzr-pull, vc-bzr-merge-branch): Use vc-set-async-update.

* vc/vc-git.el (vc-git-merge-branch): Add FETCH_HEAD to branch
completions if it exists.  Use vc-set-async-update.
(vc-git-pull): Use vc-set-async-update.

* vc/vc-hg.el (vc-hg-pull): Fix default-contents arg to
read-shell-command.  Use vc-set-async-update.
(vc-hg-merge-branch): Use vc-set-async-update.