Fix update_depot_tools for git checkout

update_depot_tools

 #!/usr/bin/env bash
 # Copyright (c) 2011 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.
 
 # This script will try to sync the bootstrap directories and then defer control.
 
 if [ "$USER" == "root" ];
 then
   echo Running depot tools as root is sad.
@@ skipping 52 matching lines @@
   fi
 }
 
 function is_git_clone_repo {
   "$GIT" config remote.origin.fetch > /dev/null
 }
 
 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
 
   test_git_svn
   # work around a git-svn --quiet bug
   OUTPUT=`"$GIT" svn rebase -q -q`
   if [[ ! "$OUTPUT" == *Current.branch* ]]; then
     echo $OUTPUT 1>&2
   fi
   return 0
@@ skipping 19 matching lines @@
   # Update the root directory to stay up-to-date with the latest depot_tools.
   BEFORE_REVISION=$(get_svn_revision)
   "$SVN" -q up "$base_dir"
   AFTER_REVISION=$(get_svn_revision)
   if [[ "$BEFORE_REVISION" != "$AFTER_REVISION" ]]; then
     echo "Depot Tools has been updated to revision $AFTER_REVISION." 1>&2
   fi
 fi
 
 find "$base_dir" -iname "*.pyc" -exec rm {} \;