Don't install new Windows git on XP machines.

bootstrap/win/win_tools.bat

 @echo off
 :: 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.
 
 :: This script will try to find if svn and python are accessible and it not,
 :: it will try to download it and 'install' it in depot_tools.
 
 :: Sadly, we can't use SETLOCAL here otherwise it ERRORLEVEL is not correctly
 :: returned.
 
 set WIN_TOOLS_ROOT_URL=https://src.chromium.org/svn/trunk/tools
 :: It used to be %~dp0 but ADODB.Stream may fail to write to this directory if
 :: the directory DACL is set to elevated integrity level.
 set ZIP_DIR=%TEMP%
 
 :: Get absolute root directory (.js scripts don't handle relative paths well).
 pushd %~dp0..\..
 set WIN_TOOLS_ROOT_DIR=%CD%
 popd
 
 if "%1" == "force" (
   set WIN_TOOLS_FORCE=1
   shift /1
 )
 
 
 :GIT_CHECK
+call ver | findstr "XP" 2>nul 1>nul
+if errorlevel 0 goto :GIT_XP_CHECK
 if "%DEPOT_TOOLS_GIT_1852%" == "0" goto :GIT_180_CHECK
 goto :GIT_1852_CHECK
 
 
 :GIT_1852_CHECK
 set GIT_VERSION=1.8.5.2.chromium.1
 set GIT_BIN_DIR=git-%GIT_VERSION%_bin
 set GIT_ZIP_FILE=%GIT_BIN_DIR%.zip
 set GIT_ZIP_URL=https://commondatastorage.googleapis.com/chrome-infra/%GIT_ZIP_FILE%
 goto :GIT_COMMON
 
 
+:: Our new build of git doesn't work on XP (uses newer APIs).
+:GIT_XP_CHECK
+:: If the new git was installed, remove all old git packages and reinstall the
+:: correct version from scratch.
+if exist "%WIN_TOOLS_ROOT_DIR%\git-1.8.5.2.chromium.1_bin" (
+  rmdir /S /Q "%WIN_TOOLS_ROOT_DIR%\git-1.8.5.2.chromium.1_bin"
+  if exist "%WIN_TOOLS_ROOT_DIR%\git-1.8.0_bin" (
+    rmdir /S /Q "%WIN_TOOLS_ROOT_DIR%\git-1.8.0_bin"
+  )
+)
+goto :GIT_180_CHECK
+
+
 :GIT_180_CHECK
 set GIT_VERSION=1.8.0
 set GIT_BIN_DIR=git-%GIT_VERSION%_bin
 set GIT_ZIP_FILE=%GIT_BIN_DIR%.zip
 set GIT_ZIP_URL=%WIN_TOOLS_ROOT_URL%/third_party/%GIT_ZIP_FILE%
 goto :GIT_COMMON
 
 
 :GIT_COMMON
 :: If the batch file exists, skip the git check.
@@ skipping 146 matching lines @@
 goto :END
 
 
 :returncode
 set WIN_TOOLS_ROOT_URL=
 set WIN_TOOLS_ROOT_DIR=
 exit /b %ERRORLEVEL%
 
 :END
 call :returncode %ERRORLEVEL%