Support for fetching git from ssh

[shonfeder]

Doing this properly will require threading the git configuration for a user's ssh key through git2

See:

• https://docs.rs/git2/0.13.17/git2/struct.Remote.html#method.connect_auth
• https://docs.rs/git2/0.13.17/git2/struct.RemoteCallbacks.html#example
• https://docs.rs/git2/0.13.17/git2/struct.Repository.html#method.config