Fix Dpkg::Changelog API usage

This fixes an old regression in tagpending where the code was switched from parsing dpkg-parsechangelog directly, to using the Dpkg::Changelog::Parse module, but didn't switch the options passed correctly. And updates the code for the upcoming code changes that deprecate the file argument (and will start emitting warnings otherwise), while preserving backwards compatibility, so that it can be deployed before the dpkg 1.23.0 upload that will have the API changes.