start_command: flush buffers in the WIN32 code path as well

The POSIX code path did The Right Thing already, but we have to do the same
on Windows.

This bug caused failures in t5526-fetch-submodules, where the output of
'git fetch --recurse-submodules' was in the wrong order.

Debugged-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.c

@@ -194,6 +194,7 @@ fail_pipe:
 	}
 
 	trace_argv_printf(cmd->argv, "trace: run_command:");
+	fflush(NULL);
 
 #ifndef WIN32
 {
@@ -201,7 +202,6 @@ fail_pipe:
 	if (pipe(notify_pipe))
 		notify_pipe[0] = notify_pipe[1] = -1;
 
-	fflush(NULL);
 	cmd->pid = fork();
 	if (!cmd->pid) {
 		/*