In dulwich.tests.test_object_store, use real sha lengths.
Strip newline from final ACKed SHA while fetching packs.

Signed-off-by: Jelmer Vernooĳ <jelmer@samba.org>
Update NEWS, add check for validity of shas passed to GraphWalker.ack.
Merge branch 'master' of git://github.com/jelmer/dulwich