submodule--helper: use "--" to signal end of clone options

When we clone a submodule, we call "git clone $url $path". But there's nothing to say that those components can't begin with a dash themselves, confusing git-clone into thinking they're options. Let's pass "--" to make it clear what we expect. There's no test here, because it's actually quite hard to make these names work, even with "git clone" parsing them correctly. And we're going to restrict these cases even further in future commits. So we'll leave off testing until then; this is just the minimal fix to prevent us from doing something stupid with a badly formed entry. Reported-by: joernchen <joernchen@phenoelit.de> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>