Enforce inserting the depot_tools path as the first path in sys.path.

git-cl-upload-hook

 #!/usr/bin/python
 # Copyright (c) 2009 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import os
 import sys
 from subprocess import Popen, PIPE
 
 # Try locating depot_tools from the user's PATH.
@@ skipping 10 matching lines @@
 if not depot_tools_path:
   # Grab a `which gclient', which gives first match
   # `which' also uses PATH, but is not restricted to specific directory name
   path = Popen(["which", "gclient"], stdout=PIPE).communicate()[0].strip()
   if path:
     depot_tools_path = path.replace("/gclient","")
 
 # If we found depot_tools, add it to the script's import path.
 # Use realpath to normalize the actual path
 if depot_tools_path:
-  sys.path.append(os.path.realpath(depot_tools_path))
+  sys.path.insert(0, os.path.realpath(depot_tools_path))
 else:
   print "ERROR: Could not find depot_tools in your PATH."
   print "ERROR: Please add it to your PATH and try again."
   sys.exit(1)
 
 # Try importing git_cl_hooks from depot_tools.
 try:
   import git_cl_hooks
 except ImportError:
   print "ERROR: Could not import git_cl_hooks from depot_tools in your PATH."
   print "ERROR: Make sure %s is up-to-date and try again." % depot_tools_path
   sys.exit(1)
 
 # Ensure we were called with the necessary number of arguments.
 program_name = os.path.basename(sys.argv[0])
 if len(sys.argv) != 2:
   print "usage: %s [upstream branch]" % program_name
   sys.exit(1)
 
 # Run the hooks library with our arguments.
 exec git_cl_hooks.RunHooks(hook_name=program_name, upstream_branch=sys.argv[1])