sync: Run gc --auto in parallel

We can't just let this run wild with a high (or even low) -j, since
that would hose a system. Instead, limit the total number of threads
across all git gc subprocesses to the number of CPUs reported by the
multiprocessing module (available in Python 2.6 and above).

Change-Id: Icca0161a1e6116ffa5f7cfc6f5faecda510a7fb9
1 file changed