WIP: do --submodule-summary using child_processes
This is super-ugly, and will suck on Windows, but Junio does not trust
my words.
This is _not_ meant for application, but just for quick-n-dirty testing.
For one, it loses coloring, and two, it loses correct determination
of fast-forwards vs non-fast-forwards, and three, it does not make the
SHA-1s being correctly abbreviated.
BUT. I have no inclination of wasting my time even more, even if
Junio asks for it. It really sucks that I have to do this.