releasing package dulwich version 0.18.1-2

debian/changelog

@@ -1,4 +1,4 @@
-dulwich (0.18.1-2) UNRELEASED; urgency=medium
+dulwich (0.18.1-2) unstable; urgency=medium
 
   * Make build-dependency on git optional, since it's not available on
     all platforms and only used for the testsuite.