Only add unstaged baseline changes to the git index when rebaselining.

third_party/WebKit/Tools/Scripts/webkitpy/common/checkout/scm/scm_mock.py

 # Copyright (C) 2011 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #    * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #    * Redistributions in binary form must reproduce the above
 # copyright notice, this list of conditions and the following disclaimer
@@ skipping 112 matching lines @@
         for path in paths:
             if self._filesystem.exists(path):
                 self._filesystem.remove(path)
 
     def move(self, origin, destination):
         if self._filesystem:
             self._filesystem.move(self.absolute_path(origin), self.absolute_path(destination))
 
     def changed_files(self):
         return []
+
+    def unstaged_changes(self):
+        return {}