convert: update use of deprecated bzrlib property

The inventory property was deprecated in favor of root_inventory in bzr
2.5.0. Current version is 2.7.0.

I noticed this when testing locally on Python 2.6.9, which has warnings
turned on by default. The failure that occurs without this patch can be
seen on Python 2.7 by running with warnings enabled:

     $ PYTHONWARNINGS=::DeprecationWarning make 'test-convert-bzr*'

hgext/convert/bzr.py

@@ -228,9 +228,10 b' class bzr_source(common.converter_source'
                     renaming = paths[0] != paths[1]
                     # neither an add nor an delete - a move
                     # rename all directory contents manually
-                    subdir = origin.inventory.path2id(paths[0])
+                    subdir = origin.root_inventory.path2id(paths[0])
                     # get all child-entries of the directory
-                    for name, entry in origin.inventory.iter_entries(subdir):
+                    for name, entry in origin.root_inventory.iter_entries(
+                            subdir):
                         # hg does not track directory renames
                         if entry.kind == 'directory':
                             continue