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/deploy/add_retry_value_to_job_event_enum.sql

@@ -0,0 +1,5 @@
+-- Deploy guix-data-service:add_retry_value_to_job_event_enum to pg
+
+ALTER TYPE job_event ADD VALUE 'retry';
+
+CREATE INDEX git_branches_name_and_datetime ON git_branches (name, datetime DESC);