Windows: a quick process no longer waits for a slow one

On Windows, a quick git call that Obeya started at the same moment as a slow one (a fetch, a push) could look unfinished until the slow one ended, because the slow process had inherited its output pipe. On the server's own thread, that stalled the whole server. Obeya now starts one process at a time on Windows, with the server's own starts first, so a quick call ends about as fast as it would alone. If a start ever hangs, the next one goes ahead after ten seconds instead of waiting for good. macOS and Linux are unaffected.