Name upstream and debian branches properly

Upstream keeps it's changes on "master". This may be the default for
`gbp`, but may *not* be. Explicitly set the upstream branch just to be
sure.

The Debian branch shouldn't be called "trunk" (or "main" or "master").
I'm calling it "deb" in case a real Debian maintainer picks up the
upstream project and wants to use the name "debian". It's configurable,
so hardly critical, but I may as well be a good neighbor and leave it
unused.

debian/gbp.conf

@@ -1,7 +1,6 @@
 [DEFAULT]
-# upstream-branch = upstream
+upstream-branch = master
-
+debian-branch = deb
-debian-branch = trunk
 upstream-tag = %(version)s
 
 pristine-tar = False