converters: Preserve GPG signatures on releases
Since version 0.19.10 (more specifically, this commit: https://github.com/dulwich/dulwich/commit/72aec0c79fb395689e40f9228df93e2a39cf8fb0), Dulwich strips GPG signatures from the 'message' attribute of Tag objects, and stores it in a new attribute, 'signature'.
This means we were silently dropping all signatures from releases.
Migrated from D6045 (view on Phabricator)