Simplify checkout slightly, replace checkdir with os.makedirs

Sort files on check-in for good I/O patterns

Make compression more intelligent: - we don't attempt to compress things under 44 bytes (empirical) - we check whether larger objects actually compress - we tag objects to indicate their compression NUL means uncompressed and starts with NUL x means gzipped and starts with x (handy) u means uncompressed, drop the u