http: inform about alternates-as-redirects behavior

It is disconcerting for users to not notice the behavior
change in handling alternates from commit cb4d2d35c4
("http: treat http-alternates like redirects")

Give the user a hint about the config option so they can
see the URL and decide whether or not they want to enable
http.followRedirects in their config.

Signed-off-by: Eric Wong <e@80x24.org>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http-walker.c

@@ -168,6 +168,11 @@ static int is_alternate_allowed(const char *url)
 	};
 	int i;
 
+	if (http_follow_config != HTTP_FOLLOW_ALWAYS) {
+		warning("alternate disabled by http.followRedirects: %s", url);
+		return 0;
+	}
+
 	for (i = 0; i < ARRAY_SIZE(protocols); i++) {
 		const char *end;
 		if (skip_prefix(url, protocols[i], &end) &&
@@ -331,9 +336,6 @@ static void fetch_alternates(struct walker *walker, const char *base)
 	struct alternates_request alt_req;
 	struct walker_data *cdata = walker->data;
 
-	if (http_follow_config != HTTP_FOLLOW_ALWAYS)
-		return;
-
 	/*
 	 * If another request has already started fetching alternates,
 	 * wait for them to arrive and return to processing this request's