Fix git depot_tools update flow.

update_depot_tools.bat

+@echo off
+:: Copyright (c) 2010 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 batch file will try to sync the root directory.
+
+:: This is required with cygwin only.
+setlocal
+PATH=%~dp0;%PATH%
+
+:: Will download svn and python.
+:: If you don't want to install the depot_tools version of these tools, remove
+:: the 'force' option on the next command. The tools will be installed only if
+:: not already in the PATH environment variable.
+call "%~dp0bootstrap\win\win_tools.bat" force
+if errorlevel 1 goto :EOF
+
+:: Shall skip automatic update?
+IF "%DEPOT_TOOLS_UPDATE%" == "0" GOTO :EOF
+
+:: We need either .\.svn\. or .\.git\. to be able to sync.
+IF EXIST "%~dp0.svn\." GOTO :SVN_UPDATE
+IF EXIST "%~dp0.git\." GOTO :GIT_UPDATE
+echo Error updating depot_tools, no revision tool found.
+goto :EOF
+
+
+:SVN_UPDATE
+call svn up -q %~dp0
+goto :EOF
+
+
+:GIT_UPDATE
+set UPDATED_DIR=%CD%
+cd %~dp0
+call git svn rebase -q -q
+cd %UPDATED_DIR%
+goto :EOF