``Branch.hooks`` now supports ``pre_commit`` hook.

The hook's signature is

::

  hook(local, master, old_revno, old_revid, new_revno, new_revid,
       deleted_paths, added_paths, future_revision_tree)

``deleted_paths`` and ``added_paths`` are lists of paths. Renamed paths are
recorded in both ``deleted_paths`` and ``added_paths`` (i.e. deleted then
added).

``future_revision_tree`` is obtained from ``CommitBuilder.revision_tree``
to save hooks from getting it from the branch again.

For example, hooks can get a file:

::

  for path in added_paths:
      id = future_revision_tree.path2id(path)
      if future_revision_tree.kind(id) == 'file':
          file = future_revision_tree.get_file(id)
          ...

or export a tree and do ``make check`` or similar

::

  import bzrlib.export
  bzrlib.export.export(future_revision_tree, 'tmp_space')


If the commit is to be rejected, hooks should raise an ``Exception``.