clean the repository

debian/gbp.conf

 [DEFAULT]
-debian-branch = debian/stretch
-upstream-branch = upstream/stretch
-debian-tag = debian/stretch/%(version)s
-upstream-tag = upstream/stretch/%(version)s
+debian-branch = debian/buster
+upstream-branch = upstream/buster
+debian-tag = debian/buster/%(version)s
+upstream-tag = upstream/buster/%(version)s
 
 [buildpackage]
-git-debian-branch = debian/stretch
-git-debian-tag = debian/stretch/%(version)s
-git-upstream-tag = upstream/stretch/%(version)s
+git-debian-branch = debian/buster
+git-debian-tag = debian/buster/%(version)s
+git-upstream-tag = upstream/buster/%(version)s
 
 [import-orig]
 pristine-tar = True