Support excluding and including branches from a repository

So that you can have the Guix Data Service only pay attention to some of the
branches.

sqitch/verify/allow_including_and_excluding_branches_for_repositories.sql

@@ -0,0 +1,7 @@
+-- Verify guix-data-service:allow_including_and_excluding_branches_for_repositories on pg
+
+BEGIN;
+
+-- XXX Add verifications here.
+
+ROLLBACK;