Rename _blob_data to blob_data

No reason I can see not to make blob_data consumable (indeed, if it
isn't, many tasks require poking at the dulwich repo directly), so
follow the naming convention to make it so.

Signed-off-by: Gregory M. Turner <gmt@be-evil.net>
Merge pull request #34 from gmt/master

Rename _blob_data to blob_data