Add extra check for bytes to read in compute_file_sha.
Add tests for adding empty thin packs to object stores.
Check for bytes rather than str.
Use bytes literals instead of string literals.
Use six.moves.
Apply libmodernize.fixes.fix_dict.

(Exclude dulwich.objects)
Use six.byte2int, .indexbytes, and .iterbytes.
Fix PackBasedObjectStore.packs.
String stuff (It's late.)
stuff