Support per repository configuration for fetching substitutes

This is so that data.qa.guix.gnu.org can be configured only to query the
branches from the main repository.

sqitch/deploy/git_repositories_query_substitutes.sql

@@ -0,0 +1,8 @@
+-- Deploy guix-data-service:git_repositories_query_substitutes to pg
+
+BEGIN;
+
+ALTER TABLE git_repositories
+  ADD COLUMN query_substitutes boolean NOT NULL DEFAULT TRUE;
+
+COMMIT;