gitea backup: prune some large directories

It's not necessary to capture the live db or git trees, so prune these
from the backups.

Change-Id: I7a27c49035eb0590d0157766eb3392a0f6331aea

inventory/service/host_vars/gitea01.opendev.org.yaml

@@ -2,3 +2,8 @@ letsencrypt_certs:
   gitea01-main:
     - gitea01.opendev.org:3000
     - opendev.org
+borg_backup_excludes_extra:
+  # git trees aren't needed
+  - /var/gitea/data/
+  # db is backed up in dumps, don't capture live files
+  - /var/gitea/db