Factor out PathDifference into gclient_utils for a later reuse.

gclient_utils.py

 # Copyright 2009 Google Inc.  All Rights Reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
 # You may obtain a copy of the License at
 #
 #      http://www.apache.org/licenses/LICENSE-2.0
 #
 # Unless required by applicable law or agreed to in writing, software
 # distributed under the License is distributed on an "AS IS" BASIS,
@@ skipping 299 matching lines @@
 
 def FindGclientRoot(from_dir):
   """Tries to find the gclient root."""
   path = os.path.realpath(from_dir)
   while not os.path.exists(os.path.join(path, '.gclient')):
     next = os.path.split(path)
     if not next[1]:
       return None
     path = next[0]
   return path
+
+def PathDifference(root, subpath):
+  """Returns the difference subpath minus root."""
+  root = os.path.realpath(root)
+  subpath = os.path.realpath(subpath)
+  if not subpath.startswith(root):
+    return None
+  # If the root does not have a trailing \ or /, we add it so the returned
+  # path starts immediately after the seperator regardless of whether it is
+  # provided.
+  root = os.path.join(root, '')
+  return subpath[len(root):]