override generation broken for gutsy

Override generation has been broken in Ubuntu gutsy since the last rollout. Specifically, many packages do not have Priority fields in the Packages files.

  $ zcat ~/ubuntu/dists/gutsy/main/binary-i386/Packages.gz | grep-dctrl -P -nsPackage -r . | wc -l
  5380
  $ zcat ~/ubuntu/dists/gutsy/main/binary-i386/Packages.gz | grep-dctrl -FPriority -nsPackage -r . | wc -l
  3623

The changes introduced to the relevant code were small (RF 4274, lib/canonical/archivepublisher/ftparchive.py), so I eyeballed them. The second hunk has a clear ordering mismatch between the addition to the SELECT clause and the addition to the sqlvalues() list. I believe that https://chinstrap.canonical.com/~cjwatson/ftparchive-overrides.diff corrects this.