There are type errors where the diff.py is making gtk2 pack_start() calls. A cursoary look at bzr-handle-patch reveals that it too is working with gtk2.
$./bzr-handle-patch ../handle-patch/_test.patch
File "./bzr-handle-patch", line 29, in <module>
controller = DiffController(path, lines, allow_dirty=True)
File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/gtk/diff.py", line 539, in __init__
window = DiffWindow(operations=self._provide_operations())
File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/gtk/diff.py", line 424, in __init__
self.construct(operations)
File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/gtk/diff.py", line 437, in construct
hbox = self._get_button_bar(operations)
File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/gtk/diff.py", line 473, in _get_button_bar
hbox.pack_start(merge_button, expand=False, fill=True)
File "/usr/lib/python2.7/dist-packages/gi/types.py", line 43, in function
return info.invoke(*args, **kwargs)
TypeError: pack_start() takes exactly 5 non-keyword arguments (2 given)
This bug was fixed in the package bzr-gtk - 0.103.0+bzr780-1
---------------
bzr-gtk (0.103.0+bzr780-1) unstable; urgency=low
* Use machine-parseable copyright file format.
* Bump standards version to 3.9.3 (no changes).
* Drop suggests of bzr-avahi, which is no longer available. LP:
#926470
* New upstream snapshot.
+ Fixes mixed use of gtk2 and gtk3 in bzr-notify. LP: #903444
+ Prevents crash when the diff widget is being closed. LP: #914363
+ Fixes syntax highlighting using the gedit color scheme. LP: #672158
+ Fixes import/type errors in bzr-handle-patch. LP: #926315
-- Jelmer Vernooij <email address hidden> Wed, 29 Feb 2012 15:04:43 +0100