Make jobs unique by commit

Given that a commit, regardless of what repository it comes from should
contain the same exactly the same data, just track jobs by commit.

sqitch/verify/load_new_guix_revision_jobs_make_commits_unique.sql

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