No more flashing terminal windows on Windows

On Windows, an Obeya started without a terminal of its own, such as the scratch instances agents start for their demos, opened a visible terminal window for every git call. Each one stole the keyboard focus, sometimes several times a second. Every program Obeya starts on Windows now runs hidden, and only the windows meant to be seen, like Explorer or the login terminal, still appear. Measured on a real desktop: dozens of windows before, none after.