mingw: factor out Windows specific environment setup
mingw: factor out Windows specific environment setup

We will add more environment-related code to that new function
in the next patch.

Signed-off-by: Karsten Blees <955b63282c975ba379c4995cac1c2e1e7dfebe62@dcon.de>
Signed-off-by: Johannes Schindelin <53fb8db7833fca1e1746dd8592f61048d350f64c@gmx.de>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git