Update Dulwich, remove duplicate code

Update to a newer version of Dulwich, and use functionality from upstream Dulwich where possible. 

This allows dropping of the copy of configobj that rabbitvcs was shipping.
That is an easy thing for me to do. I am upgrading my packaging machine this week, and on the weekend I was going to taking in recent changes to packaging, make sure the Utopic packages are good, and update the PPAs. I can include this too.