At some point git-buildpackage was not used anymore for this repository
(see [1]). For example, the upstream branch was edited manually. That is
a problem, because these changes will not be in the package. In these
packaging git repositories, the upstream branch is only changed via git
import-orig. This is desribed for instance in [2]. Changes to the
sources have to be done by patches in debian/patches, usually using
quilt [3]. The repository can probably be fixed by doing a git
import-orig with the current orig tarball.