def write_source_patches
      from_directory do
        id = @git.latest_id("tetra: sources-tarball")
        destination_path = File.join(full_path, packages_dir, name)
        @git.format_patch("src", id, destination_path)
      end
    end
def write_source_patches
      from_directory do
        id = @git.latest_id("tetra: sources-tarball")
        destination_path = File.join(full_path, packages_dir, name)
        @git.format_patch("src", id, destination_path)
      end
    end
generates patches of src/ in packages/
 the latest commit marked as tarball is taken as the base version,
 other commits are assumed to be patches on top
 returns filenames