debian: Add gbp dch configuration

Configure gbp dch according to
https://phabricator.apertis.org/w/packages/git_flow/



Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>

debian/gbp.conf

 [DEFAULT]
 debian-tag = %(version)s
+
+[dch]
+ignore-branch = True
+meta-closes = Apertis
+meta-closes-bugnum = (?:https://phabricator.apertis.org/)?(\T?\d+)