Be protocol-agnostic wrt/ Git URL's

Today's sub-module URL scheme is `git://` regardless of whether the top-
level (gitflow) repository.  is SSH (`git@github:...), HTTPS
(`https://`) or Git socket (`git://`).

This small change uses relative URL's for sub-modules & hence re-use
the top-level repo URL model for the sub-module.
Merge pull request #1 from snowfix/feature/clone-from-any-url-protocol

Be protocol-agnostic wrt/ Git URL's