Fix broken slot reuse when fetching alternates

When fetching alternates, http-fetch may reuse the slot to fetch non-http
alternates if http-alternates does not exist.  When doing so, it now needs
to update the slot's finished status so run_active_slot waits for the
non-http alternates request to finish.

Signed-off-by: Nick Hengeveld <nickh@reactrix.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

http-fetch.c

@@ -468,9 +468,13 @@ static void process_alternates_response(void *callback_data)
 					 alt_req->url);
 			active_requests++;
 			slot->in_use = 1;
+			if (slot->finished != NULL)
+				(*slot->finished) = 0;
 			if (!start_active_slot(slot)) {
 				got_alternates = -1;
 				slot->in_use = 0;
+				if (slot->finished != NULL)
+					(*slot->finished) = 1;
 			}
 			return;
 		}