Fixed double separators appearing in prefab instances

Flatten is called twice for prefab instances present in the scene: first when prefab is edited, and second when the scene hierarchy is processed. This caused double --- wrappers appearing when "Replace with separator" option was chosen.

Author: SolidAlloy

Runtime/Folder.cs

@@ -222,7 +222,12 @@ private void HandleSelf(StrippingMode strippingMode, bool capitalizeFolderName)
         {
             if (strippingMode == StrippingMode.ReplaceWithSeparator)
             {
-                name = $"--- {(capitalizeFolderName ? name.ToUpper() : name)} ---";
+                // If the folder name is already a separator, don't change it.
+                if ( ! name.StartsWith("--- "))
+                {
+                    name = $"--- {(capitalizeFolderName ? name.ToUpper() : name)} ---";
+                }
+
                 return;
             }