minibar/support.js: recognize relative URLs as locations on the same server

This means that the relative grammar directory location in the default
configuration will result in AJAX requests rather than the less efficient
JSONP requests.

src/runtime/javascript/minibar/support.js

@@ -90,7 +90,9 @@ function ajax_http_get_json(url,cont) {
 }
 
 function sameOrigin(url) {
-    return hasPrefix(url,location.protocol+"//"+location.host+"/");
+    var a=empty("a");
+    a.href=url; // converts to an absolute URL
+    return hasPrefix(a.href,location.protocol+"//"+location.host+"/");
 }
 
 // Use AJAX when possible, fallback to JSONP