add v6; keep v5 working for now and manual upgrade

add v6; keep v5 working for now and manual upgrade

Since all places where a repo is used in direct mode need to have git-annex
upgraded before the repo can safely be converted to v6, the upgrade needs
to be manual for now.

I suppose that at some point I'll want to drop all the direct mode support
code. At that point, will stop supporting v5, and will need to auto-upgrade
any remaining v5 repos. If possible, I'd like to carry the direct mode
support for say, a year or so, to give people plenty of time to upgrade and
avoid disruption.