fix bug where dir gets skipped when switching Unity project

- in changing head, then changing tail, one more directory gets dropped because tail is now taken from the updated $head value

Author: vkovec

Assets/com.unity.formats.fbx/Editor/Integrations/Autodesk/maya/scripts/unityCommands.mel

@@ -320,8 +320,8 @@ proc switchUnityProject(string $newProjectPath){
             optionVar -sv "UnityProject" $head;
             break;
         }
-        $head = dirname($head);
         $tail = basename($head, "");
+        $head = dirname($head);
     }
 }