Fix two more leaking tmp dirs, by reworking TransformPreview lock handling.

* bzrlib/tests/test_transform.py:
(TestTransformMerge): Revert previous patch and cleanly call
preview.finalize now that we can.

* bzrlib/tests/test_merge.py:
(TestMerge.test_make_preview_transform): Catch TransformPreview
leak.

* bzrlib/builtins.py:
(cmd_merge._do_preview): Finalize the TransformPreview or the
limbodir will stay in /tmp.

* bzrlib/transform.py:
(TreeTransformBase.__init__): Create the _deletiondir since it's
reffered to by finalize.
(TreeTransformBase.finalize): Delete the dir only if _deletiondir
is set.
(TreeTransform.__init__): Use a temp var for deletiondir and set
the attribute after the base class __init__ has been called.
(TransformPreview.__init__): Read locks the tree since finalize
wants to unlock it (as suggested by Aaron).