Generate rollback delta in a test subdirectory

Test job https://jenkins.apertis.org/job/playground/job/fredo/job/build-ostree/55/

and its result: https://images.apertis.org/test/playground/fredo/build-ostree/daily/v2019dev0/20190321.0/armhf/minimal/

Signed-off-by: Frédéric Dalleau frederic.dalleau@collabora.com