update config

debian/gbp.conf

 [DEFAULT]
+debian-branch = debian/stretch
+upstream-branch = upstream/stretch
+debian-tag = debian/stretch/%(version)s
+upstream-tag = upstream/stretch/%(version)s
 
-# The default name for the upstream branch is "upstream".
-# Change it if the name is different (for instance, "master").
-upstream-branch = upstream
-
-# The default name for the Debian branch is "master".
-# Change it if the name is different (for instance, "debian/unstable").
-debian-branch = master
-
-# git-import-orig uses the following names for the upstream tags.
-# Change the value if you are not using git-import-orig
-upstream-tag = upstream/%(version)s
+[buildpackage]
+git-debian-branch = debian/stretch
+git-debian-tag = debian/stretch/%(version)s
+git-upstream-tag = upstream/stretch/%(version)s
 
-# Always use pristine-tar.
+[import-orig]
 pristine-tar = True
-
-[buildpackage]
-pbuilder-options = --source-only-changes