transform.final_kind() now returns None instead of raising NoSuchFile.

* bzrlib/transform.py:
(TreeTransformBase.final_kind): Just return None instead of
raising NoSuchFile since all callers just do that while catching
the exception.
(TreeTransformBase._improper_versioning)
(TreeTransformBase._executability_conflicts)
(TreeTransformBase._duplicate_entries)
(TreeTransformBase._parent_type_conflicts)
(TreeTransformBase._any_contents, TreeTransformBase._to_file_data)
(TreeTransform._generate_inventory_delta)
(_PreviewTree._make_inv_entries, _PreviewTree.walkdirs): Fix call
sites.

* bzrlib/tests/test_transform.py:
(TestTreeTransform.test_build): The single test needing a fix for
final_kind returning None...

* bzrlib/merge.py:
(Merge3Merger.fix_root, Merge3Merger.fix_root)
(Merge3Merger._merge_executable): Simplify final_kind() uses.