Add an installer for the msysgit environment

This installer:
- detects the Git installation path
- checks prereqs (git, getopt.exe)
- delete old gitflow install
- install gitflow files

"Access denied" errors are detected and the user is given advice in that case (
use full administrator rights).

contrib/msysgit-install.cmd

+@echo off
+setlocal
+if not "%~1"=="" set GIT_HOME=%~f1
+if "%GIT_HOME%"=="" call :FindGitHome "git.cmd"
+
+if exist "%GIT_HOME%" goto :GitHomeOK
+
+echo MsysGit installation directory not found.>&2
+echo Try to give the directory name on the command line:>&2
+echo   %0 "%ProgramFiles%\Git"
+endlocal
+exit /B 1
+
+:GitHomeOK
+set ERR=0
+
+echo Installing gitflow into "%GIT_HOME%"...
+
+call :ChkGetopt getopt.exe || set ERR=1
+if %ERR%==1 goto :End
+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
+
+echo Deleting old files...
+for /F %%i in ("%GIT_HOME%\git-flow*" "%GIT_HOME%\gitflow-*") do if exist "%%~fi" del /F /Q "%%~fi"
+
+:Install
+echo Copying files...
+::goto :EOF
+xcopy "%~dp0\..\git-flow"            "%GIT_HOME%\bin"                 /Y /R /F
+if errorlevel 4 if not errorlevel 5 goto :AccessDenied
+if errorlevel 1 set ERR=1
+xcopy "%~dp0\..\git-flow*"           "%GIT_HOME%\bin"                 /Y /R /F || set ERR=1
+xcopy "%~dp0\..\gitflow-*"           "%GIT_HOME%\bin"                 /Y /R /F || set ERR=1
+xcopy "%~dp0\..\shFlags\src\shflags" "%GIT_HOME%\bin\gitflow-shFlags" /Y /R /F || set ERR=1
+
+if %ERR%==1 choice /T 30 /C Y /D Y /M "Some unexpected errors happened. Sorry, you'll have to fix them by yourself."
+
+:End
+endlocal & exit /B %ERR%
+goto :EOF
+
+:AccessDenied
+set ERR=1
+echo.
+echo You should run this script with "Full Administrator" rights:>&2
+echo - Right-click with Shift on the script from the Explorer>&2
+echo - Select "Run as administrator">&2
+choice /T 30 /C YN /D Y /N >nul
+goto :End
+
+:Abort
+echo Installation canceled.>&2
+set ERR=1
+goto :End
+
+:ChkGetopt
+:: %1 is getopt.exe
+if exist "%GIT_HOME%\bin\%1" goto :EOF
+if exist "%~f$PATH:1" goto :EOF
+echo %GIT_HOME%\bin\%1 not found.>&2
+echo You have to install this file manually. See the GitFlow README.
+exit /B 1
+
+:FindGitHome
+setlocal
+set GIT_CMD_DIR=%~dp$PATH:1
+if "%GIT_CMD_DIR%"=="" endlocal & goto :EOF
+endlocal & set GIT_HOME=%GIT_CMD_DIR:~0,-5%
+goto :EOF