Allow specifying the remote helper in the url

The common case for remote helpers will be to import some repository
which can be specified by a single URL.  Support this use case by
allowing users to say:

	git clone hg::https://soc.googlecode.com/hg/ soc

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

transport.c

@@ -818,6 +818,16 @@ struct transport *transport_get(struct remote *remote, const char *url)
 		url = remote->url[0];
 	ret->url = url;
 
+	/* maybe it is a foreign URL? */
+	if (url) {
+		const char *p = url;
+
+		while (isalnum(*p))
+			p++;
+		if (!prefixcmp(p, "::"))
+			remote->foreign_vcs = xstrndup(url, p - url);
+	}
+
 	if (remote && remote->foreign_vcs) {
 		transport_helper_init(ret, remote->foreign_vcs);
 		return ret;