Drop unstable branch, just use 'master' per DPMT policy.

debian/control

@@ -8,7 +8,7 @@ Build-Depends: python-all-dev (>= 2.6.6-3), python-all-dbg (>= 2.6.6-3), python3
 Standards-Version: 3.9.6
 X-Python-Version: >= 2.7
 X-Python3-Version: >= 3.2
-Vcs-Git: git://anonscm.debian.org/python-modules/packages/dulwich.git -b unstable
+Vcs-Git: git://anonscm.debian.org/python-modules/packages/dulwich.git
 Vcs-Browser: http://anonscm.debian.org/gitweb/?p=python-modules/packages/dulwich.git
 XS-Testsuite: autopkgtest