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>

compat/mingw.c

@@ -3086,6 +3086,10 @@ static void maybe_redirect_std_handles(void)
 
 #if defined(_MSC_VER)
 
+#ifdef _DEBUG
+#include <crtdbg.h>
+#endif
+
 /*
  * This routine sits between wmain() and "main" in git.exe.
  * We receive UNICODE (wchar_t) values for argv and env.
@@ -3110,6 +3114,10 @@ int msc_startup(int argc, wchar_t **w_argv, wchar_t **w_env)
 	int maxlen;
 	int k, exit_status;
 
+#ifdef _DEBUG
+	_CrtSetReportMode(_CRT_ASSERT, _CRTDBG_MODE_DEBUG);
+#endif
+
 #ifdef USE_MSVC_CRTDBG
 	_CrtSetDbgFlag(_CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF);
 #endif