Fix update_depot_tools for git checkout

Otherwise, the output was:
  fatal: Needed a single revision
  invalid upstream origin

R=szager@chromium.org
BUG=
TEST=


Review URL: https://chromiumcodereview.appspot.com/10784045

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@147064 0039d316-1c4b-4281-b951-d872f2087c98

update_depot_tools

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-# Copyright (c) 2011 The Chromium Authors. All rights reserved.
+# Copyright (c) 2012 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.
 
@@ -70,7 +70,7 @@ function is_git_clone_repo {
 function update_git_repo {
   if is_git_clone_repo; then
     git fetch -q origin > /dev/null
-    git rebase -q origin > /dev/null
+    git rebase -q origin/master > /dev/null
     return 0
   fi