Revert default Windows git to the old version.

There are several bot failures, some understood, some not. This will revert
bots to the old git until all the failures can be investigated.

Users who want the new git can still install it with an explicit:
set DEPOT_TOOLS_GIT_1852=1

R=hinoka@google.com

Review URL: https://codereview.chromium.org/150203005

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@250239 0039d316-1c4b-4281-b951-d872f2087c98

bootstrap/win/win_tools.bat

@@ -26,10 +26,11 @@ if "%1" == "force" (
 
 
 :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
+if "%DEPOT_TOOLS_GIT_1852%" == "1" goto :GIT_1852_CHECK
+:: Our new build of git doesn't work on some systems (e.g. uses newer APIs than
+:: available in XP), so force systems back to the old git while the failures
+:: are investigated, unless the new git was explicitly requested above.
+goto :GIT_180_FORCE
 
 
 :GIT_1852_CHECK
@@ -40,10 +41,9 @@ set GIT_ZIP_URL=https://commondatastorage.googleapis.com/chrome-infra/%GIT_ZIP_F
 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.
+:GIT_180_FORCE
+:: If the new git was installed, remove all installed git packages to trigger
+:: reinstallation of the old version.
 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" (