hashsplit.py is now much, much faster than before.

hashsplit.py is now much, much faster than before.

4.8 secs vs. 0.8 secs for testfile1.

Still vastly slower than the C version (0.17 secs including time to fork
git for each blob) but still a significant improvement.

The remaining slowness seems to be entirely from:

- running git hash-object (which we can avoid by hashing the object
  ourselves)

- running the rolling checksum algorithm (which we can speed up using a C
  module)

So it's looking good.