add FindPatch.cmake

I've added FindPatch.cmake. This is only used when compiling under MSVC. CMake will exit with an error if patch.exe is not found (it'll search in system paths and the default installation directories of msysGit). I've also removed tortoiseGit as it's unneeded (msysGit provides both Git and patch.exe).

This pull needs to be tested on Linux/mingw as I made some changes that could affect these platforms.