Clone as non-bare repository to cache submodules (performance up)

I tried Mina in my project including big submodule (WordPress).
But it takes too much time to clone submodules every deployment.
So I propose that it keeps non-bare repository with submodules like Capistrano's remote-cache.

referred to: https://github.com/capistrano/capistrano/blob/master/lib/capistrano/recipes/deploy/scm/git.rb

This would add an additional dependency for the server side.
Maybe this step could be done without `rsync`?