Make Geo use Git streaming backups

I was pointed out by @zj-gitlab the Gitaly teams considers/is building streaming Git backups, see: &2094. So they want to create incremental backup files which can be replayed to rebuild a repo from Object Storage.

In the future, we can consider also using these backups to replicate repos between Geo nodes. It would avoid putting any stress on the primary.

It would also improve the initial replication, because the secondary can just load all files from OS and rebuild the repo quite quickly.