mergechanges: Merge Binary and Description fields (#920470) and add a test

Recent dpkg versions only emit applicable packages in the Binary and Description fields, omitting the field entirely if it would be empty, so we now need to filter those fields to include only those packages that are to be uploaded with the changes file.

This also adds a basic regression test. The .changes files used here are real data from my recent xdg-desktop-portal upload, as produced with dpkg (>= 1.19.3) and sbuild. In particular, the source changes file doesn't have Binary or Description fields. I've left the signature on the source changes file, so that it can be used to verify that merging a signed changes file results in unsigned output.