Fix changeset fetching
Fix changeset fetching

This change adds support for fetching over the anonymous git and
authenticated SSH protocols, and it also makes sure that we follow URLs
specified by the download plugin (they might differ from the server's
address even for anonymous HTTP).

Also see https://storyboard.openstack.org/#!/story/294 .

Change-Id: Iafb7ae63147fc7a70ab63a9d6f61617b87e65ac9

stackforge/gertty,openstack/gertty,aspiers/gertty