debian/gbp.conf: adjust branch and tags.

Signed-off-by: Andrew Lee (李健秋) <ajqlee@debian.org>

debian/gbp.conf

 [DEFAULT]
-debian-branch = debian/master
-debian-tag = debian/%(version)s
+debian-branch = collabora/master
+debian-tag = collabora/%(version)s
 pristine-tar = True
 
 [pq]