AFAICs there's no need to run 'git submodule init'
Add GITHUB_SSH_HOST_KEY
git_clone() makes sure the github SSH host key is known
Smarter git_clone()

Play games with SSH_AUTH_SOCK if and only if checking out a repository
over SSH.

Add the SSH host key to root's known_hosts if and only if checking out
a repository over SSH from a host in KNOWN_HOSTS (currently this means
GitHub).

Add some tests because I can't write helpers like parse_git_repo()
without tests.