bundle2: fix bundle2 pulling all revs on empty pulls When bundle2 was enabled, if hg pull had no commits to pull, it would print 'no changes found' and then download the entire repository from the server. This was caused by heads and common being set to None, which gets treated as heads=cl.heads() and common=[nullid], which means download the entire repo. Pulling bundles without a changegroup is a valid use case (like if we're just updating bookmarks), so this modifes the bundle code to allow not adding changegroups.