Don't install new Windows git on XP machines.

bootstrap/win/win_tools.bat

Index: bootstrap/win/win_tools.bat
===================================================================
--- bootstrap/win/win_tools.bat	(revision 250181)
+++ bootstrap/win/win_tools.bat	(working copy)
@@ -26,6 +26,8 @@
 
 
 :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
 
@@ -38,6 +40,19 @@
 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