get actual cpu count instead of just launching 8 processes.

[matthiaskrgr]

I noticed that gitstats would launch 8 processes on my dualcore machine.
My commit changes it so that we only launch as much processes as we actually have cores.

[hoxu]

Hello Matthias,

Did you actually run (clean) benchmarks to find out the optimal amount
of processes for your setup?

I dug up some old e-mail threads from when multiprocessing was added,
and my personal finding was that the performance was not optimal with
1:1 cores:processes ratio at least :)

Probably because a lot of stuff is not cpu-bound, but I haven't dug
deeper to really understand where the bottleneck usually is.

Would be nice to see more empirical data before changing the default.
Possibly processes = 2*cores could be a good default.

Regards,

Heikki Hokkanen