better workflow for getting all the relevant branches

[Martin Pool]

https://lists.ubuntu.com/archives/ubuntu-distributed-devel/2010-January/000373.html

1. The most important change would be to have some kind of a wrapper for
getting the source. It would be nice to have a script that would download
branches for the common ancestor, current Ubuntu, and current Debian branches,
and do the proposed merge. Without local access to the different versions of
the package, it is very difficult to know if you've got a correct merge.

Additionally, if there were checkouts for the previous Debian and Ubuntu
packages locally, then it should be easy enough to diff the debian directories
to check for packaging changes when a new upstream version is involved.