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.
On Mon, Aug 25, 2014 at 12:54:44AM -0700, d❤vid seaward wrote:
> > @@ -24,7 +24,7 @@ For subversion:
> >      subversion >= 1.4.6
> >  
> >  For git:
> > -    dulwich >= 0.6.1
> > +    dulwich >= 0.9.7
> 
> @jelmer and @CloCkWeRX just a note that in Ubuntu 14.04 only dulwich 0.9.4 (called `python-dulwich`) is available. Not sure if the solution is to downgrade this requirement, add dulwich 0.9.7 to RabbitVCS PPA or something else?

For most things, 0.9.4 should be sufficient as well but it would be
great to add 0.9.7 to the PPA - it has a number of bug fixes and
performance improvements that would be useful for rabbitvcs.

dulwich 0.9.7 is already packaged in Debian, so it should be possible
to just copy the package over from Debian to the PPA.

Jelmer

-- 
Jelmer Vernooij <jelmer@samba.org> - https://jelmer.uk/