Use gbp-buildpackage as example

Instead of using git example, use gbp's

Our own tools, import-debian-package for example, all use
gbp-buildpackage as the intermediary than using git directly.

The other perk of using gbp-buildpackage is that it takes care of
nuances like .gitattributes and upstream debian/ folders.

Signed-off-by: Ritesh Raj Sarraf <ritesh.sarraf@collabora.com>