Route non-native URL schemes through git remote helpers

newConn now checks for a git-remote-<scheme> binary when the endpoint scheme isn't one git-sync handles natively, and builds a HelperConn when one exists. http/https are excluded so they always use the optimized native HTTP transport (git ships its own git-remote-http(s)); ssh is already handled above. This is what makes entire:// URLs work, given git-remote-entire on PATH.