CloneOptions should reuse Auth method when recursing submodules

[devonbarrett]

_, err := git.PlainClone(pathToService, false, &git.CloneOptions{
	URL:               "ssh://git@host:repo.git",
	Auth:              sshAuth,
	RecurseSubmodules: 10,
})
// err = error creating SSH agent: "SSH agent requested but SSH_AUTH_SOCK not-specified"

Observed

The PlainClone of the initial repository succeeds, however recursing submodules fail as they require use of the same transport.AuthMethod

Expected

The Auth method set in the CloneOptions is also used as a part of the SubmoduleUpdateOptions.Auth

Happy to make a PR if that is desirable?

[mcuadros]

sure