branch.py:

   * ``pre_commit`` hook's signature is changed to::

   hook(local, master, old_revno, old_revid, future_revno, future_revid,
        tree_delta, future_tree)

   * made it clear that pre_commit hooks must not modify neither
     the delta nor future_tree

commit.py:

   * replaced ``affected_ids`` with ``tree_delta``

   * only compute ``future_tree`` and ``tree_delta`` once