Script sometimes gets confused by full removal

[jwir3]

When the diff file has input like the following:

diff --git a/dom/interfaces/canvas/nsIDOMCanvasRenderingContext2D.idl b/dom/interfaces/canvas/nsIDOMCanvasRenderingContext2D.idl
--- a/dom/interfaces/canvas/nsIDOMCanvasRenderingContext2D.idl                  
+++ b/dom/interfaces/canvas/nsIDOMCanvasRenderingContext2D.idl                  
@@ -1,32 +1,15 @@                                                               
 /* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* This Source Code Form is subject to the terms of the Mozilla Public         
  * License, v. 2.0. If a copy of the MPL was not distributed with this         
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */                 

 #include "nsISupports.idl"                                                     

-[scriptable, uuid(bbb20a59-524e-4662-981e-5e142814b20c)]                       
-interface nsIDOMCanvasGradient : nsISupports                                   
-{                                                                              
-  void addColorStop(in float offset, in DOMString color);                      
-};                                                                             
-                                                                               
-[scriptable, uuid(21dea65c-5c08-4eb1-ac82-81fe95be77b8)]                       
-interface nsIDOMCanvasPattern : nsISupports                                    
-{                                                                              
-};                                                                             
-                                                                               
-[scriptable, uuid(2d01715c-ec7d-424a-ab85-e0fd70c8665c)]                       
-interface nsIDOMTextMetrics : nsISupports                                      
-{                                                                              
-  readonly attribute float width;                                              
-};                                                                             
-                

The script incorrectly reports these interfaces as needing IID changes. Any time an interface has been completely removed from a file, the script should identify this as not needing an IID change.