Code review comment for lp:~jelmer/bzr-builddeb/obj

The attached patch adds a new class UpstreamSource (for lack of a
better name) that represents a source of upstream tarballs.
UpstreamSources currently only can retrieve specific version tarballs,
but the idea is that they'll in the future also be able to (where
possible/applicable):

 * retrieve the last upstream version string
 * retrieve the last upstream version tarball
 * return the revid matching a particular upstream version
 * list the available upstream versions

It also makes bzr-builddeb support retrieval of upstream snapshot
versions again ("1.0~bzr13").

Cheers,

Jelmer