Find already installed git if exists, and use it instead of embeded one in Cmder

Works for default path, 32/64 bits.
Really useful when git is installed an updated with Chocolatey.
I've added it so the PATH will be the same.
I think bliker added it in order to be able to run `cmder.exe /REGISTER ALL*` from Cmder itself (without it, you need to navigate to the right directory).
See commit 9939b1ba7f0aa60507fca81c5bf39300b40ba833