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/verify/add_retry_value_to_job_event_enum.sql

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