fixed the installer in msysgit environment to work with windows XP

as the command 'choice' is not available on Windows XP we have to work around
this issue by usage of 'set /p'
diff --git a/contrib/msysgit-install.cmd b/contrib/msysgit-install.cmd
index 1285839..5f005bf 100644
--- a/contrib/msysgit-install.cmd
+++ b/contrib/msysgit-install.cmd
@@ -22,11 +22,11 @@
 

 if not exist "%GIT_HOME%\bin\git-flow" goto :Install

 echo GitFlow is already installed.>&2

-choice /C YN /M "Do you want to replace it"

-if errorlevel 255 goto :Abort

-if errorlevel 2 goto :Abort

-if not errorlevel 1 goto :Abort

+set /p mychoice="Do you want to replace it [y/n]"

+if "%mychoice%"=="y" goto :DeleteOldFiles

+goto :Abort

 

+:DeleteOldFiles

 echo Deleting old files...

 for /F %%i in ("%GIT_HOME%\git-flow*" "%GIT_HOME%\gitflow-*") do if exist "%%~fi" del /F /Q "%%~fi"