deb: change source format to "3.0 (quilt)"
deb: change source format to "3.0 (quilt)"

Previously we used "3.0 (git)", but this was not officially supported
(i.e. we probably just invented it).  Now dpkg-buildpackage seems to
start to support it and thus the build fails.

stadtgestalten/stadtgestalten,stadtgestalten/stadtgestalten,stadtgestalten/stadtgestalten