Add cron to gc on gitea servers

As new change refs accumulate, replication pushes and page loads
will take longer as git stats all of the refs/ files.  To avoid
that, pack refs and gc every week to keep the number of files
and space used minimal.

Change-Id: Iff273ebbc25a512ab7e12b8418ceb30e7c722f92

playbooks/roles/gitea/tasks/main.yaml

@@ -126,3 +126,11 @@
       key: "{{ gitea_gerrit_public_key }}"
       read_only: false
       title: "Gerrit replication key"
+- name: Set up cron job to pack git refs
+  cron:
+    name: pack-git-refs
+    state: present
+    job: "docker exec -t giteadocker_gitea-web_1 find /data/git/repositories/ -maxdepth 2 -name *.git -type d -execdir git --git-dir={} gc \\;"
+    minute: 37
+    hour: 12
+    weekday: 0