Make it easier to retry jobs

Add a new event 'retry', and run jobs where the number of retry events is
greater or equal to the number of failure events.

Also add an index to the git_branches table to make the finding jobs query a
bit faster.

sqitch/revert/add_retry_value_to_job_event_enum.sql

@@ -0,0 +1,7 @@
+-- Revert guix-data-service:add_retry_value_to_job_event_enum from pg
+
+BEGIN;
+
+-- XXX Add DDLs here.
+
+COMMIT;