mingw32-git: Use relative path for cmd.exe post-install action

This way it is easier to call git.exe later.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
mingw32-git: Silence the robocopy post-install action a bit

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>