Diff generation in the proposed pocket should consider the updates pocket even when there are previous proposed publications.

[Dave Walker]

Consider https://launchpad.net/ubuntu/+source/dpkg/1.15.5.6ubuntu4.4

The LP generated diff is against the ubuntu4.2 version *not* the ubuntu4.3 version.

This would appear to be because LP has preferred to use the previous published version in the proposed pocket, but would ideally use the newer publication present in the updates pocket (which entered there from the security pocket, and therefore was never published in the proposed poclet.)

[Dave Walker (davewalker)]

Another example:
http://launchpadlibrarian.net/59609909/libvirt_0.7.5-5ubuntu27.3_0.7.5-5ubuntu27.8.diff.gz

[Julian Edwards (julian-edwards)]

When looking for the previous version, we should be more intelligent about which pockets to use as per the pocket_dependencies dictionary in lib/lp/soyuz/adapters/archivedependencies.py