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'

contrib/msysgit-install.cmd

@@ -22,11 +22,11 @@ echo getopt.exe... Found
 
 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"