'bup split': speed optimization for never-ending blocks.

'bup split': speed optimization for never-ending blocks.

For blocks which never got split (eg. huge endless streams of zeroes) we
would constantly scan and re-scan the same sub-blocks, making things go
really slowly.  In such a bad situation, there's no point in being so careful;
just dump the *entire* input buffer to a chunk and move on.  This vastly
speeds up splitting of files with lots of blank space in them, eg.
VirtualBox images.

Also add a cache for git.hash_raw() so it doesn't have to stat() the same
blob files over and over if the same blocks (especially zeroes) occur more
than once.