Allow to build GNOME packages from a patched branch

Maintaining debian/patches by hand (even with tool support) is cumbersome. Tools that manage patched trees yield to complex histories confusing people as well. So for CI use gbp's sloppy build feature to create new upstream tarballs for the patched tree (which we can do since Debian's GNOME team uses packaging based on upstream git).

Closes: #11 (closed)