Record the bzr branch and revision that a source package was uploaded from.

Currently we have no link back to the bzr branch that distro packages are built from. A small amount of work will create a tool which records the public url of the branch and the revision on the branch during package building. A similarly small amount of work can capture this information during the upload process and insert it into launchpad. The end result will be a link from sourcepackagereleases to the branch that the release came from when it was released from a branch.

Things we need to watch out for:
 * If we see a duplicate upload for a package - with the same branch and revision id - we should ignore the branch and revision id unless the debian version is identical to the previous version. (that duplication may be an error anyway)... This handles a non-bzr enabled person doing an upload.
 * The tool that saves the bzr branch data to debian/branch should ensure that 'bzr st' returns 0, and 'bzr ls --unknowns' returns an empty list. This is to ensure that there is no cruft in the directory - perhaps it can be optional.