msvc: avoid debug assertion windows in Debug Mode

For regular debugging, it is pretty helpful when a debug assertion in a running application triggers a window that offers to start the debugger. However, when running the test suite, it is not so helpful, in particular when the debug assertions are then suppressed anyway because we disable the invalid parameter checking (via invalidcontinue.obj, see the comment in config.mak.uname about that object for more information). So let's simply disable that window in Debug Mode (it is already disabled in Release Mode). Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>