http: allow proxies to be specified in common format

The common format for specifying proxy URLs is just 'host:port'. Handle
the common case.

src/libgit2/transports/http.c

@@ -335,7 +335,7 @@ static int lookup_proxy(
 	}
 
 	if (!proxy ||
-	    (error = git_net_url_parse(&transport->proxy.url, proxy)) < 0)
+	    (error = git_net_url_parse_http(&transport->proxy.url, proxy)) < 0)
 		goto done;
 
 	if (!git_net_url_valid(&transport->proxy.url)) {