Allow git-fat settings to come from global git configuration

Being able to configure settings like rsync.remote on a per-repository basis is great.

However, I'd also like the ability to configure settings like rsync.sshuser on a per user basis. For example, my sshusername might be chrismarinos, but yours might be jedbrown. This is especially handy when your sshuser is different from your local machine's user name.

This patch makes that possible.