Fix release message body

[jannis-baum]

Close #113

[jannis-baum]

@tuurep Hey so a little update on the release: It almost worked haha. Only thing was that the tag's message didn't properly get inserted into the release notes, this should fix it now☺️

Problems were

1. We needed to use just ref_name and not the full ref with the command
2. The content:subject for whatever reason deletes all line breaks from the message so we have to get all content which ends in a PGP signature that we delete with sed