Switch to storing Git repositories in a table

Rather than just storing the URL in the guix_revisions and
load_new_guix_revision_jobs tables. This will help when storing more
information like tags and branches in the future.

sqitch/verify/git_repositories.sql

@@ -0,0 +1,8 @@
+-- Verify guix-data-service:git_repositories on pg
+
+BEGIN;
+
+SELECT id, label, url
+  FROM git_repositories WHERE FALSE;
+
+ROLLBACK;