Fix index.changes_from_tree against an empty tree.

index.changes_from_tree(object_store, None) should work. Modify that function
so the 'tree' argument can be None to signify an empty tree.

Signed-off-by: Jelmer Vernooij <jelmer@samba.org>
Update NEWS.
Check for empty-repo on client fetch

An empty repo will respond with the flush-pkt only.

Signed-off-by: Jelmer Vernooij <jelmer@samba.org>
Update NEWS, AUTHORS.