hashsplit: smallish files (less than BLOB_MAX) weren't getting split.

hashsplit: smallish files (less than BLOB_MAX) weren't getting split.

This buglet was introduced when doing my new fanout cleanups.  It's
relatively unimportant, but it would cause a bit of space wastage for
smallish files that changed by a bit, since we couldn't take advantage of
deduplication for their blocks.

This also explains why the --fanout argument test broke earlier.  I thought
I was going crazy (since the whole fanout implementation had changed and the
number now means something slightly different), so I just removed it.  But
now we can bring it back and it passes again.^