update remote on ref-update

Whenever receiving a ref-update event, for example when a tag is
created, deleted or changed, the local Zuul git repository would not
reflect the change.

This patch force a git remote update on the local git repository so the
Jenkins git plugin can actually find the new tag. This is done before
enqueing the change in the pipeline to only update the remote once.

Change-Id: I87a29aa15855bfb367f03da3e15cd8a733b757fd
Reviewed-on: https://review.openstack.org/19928
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins