Merge lp://staging/~sinzui/bzr-gtk/precise-diff-0 into lp://staging/bzr-gtk

Do not update the DiffWidget when it is being destroyed.

This is a fix for bug 914363, which is private so I cannot see it.
It may not even be reported against bzr-gtk. Everyone can see my
bug 920525 that Apport says is a duplicate.

This is a bzr-gtk gdiff issue. The diff works, but an error happens when
the window is closed. This may be the same kind of issue I fixed in
gcommit where the callback is called while the window is being
destroyed. In that case, The treeview still existed, but it's model and
state were None.

Traceback (most recent call last):
  File "/home/curtis/.bazaar/plugins/gtk/diff.py", line 391, in _treeview_cursor_cb
    specific_files = [ self.model[path][1] ]
  File "/usr/lib/python2.7/dist-packages/gi/overrides/Gtk.py", line 730, in __getitem__
    aiter = self.get_iter(key)
  File "/usr/lib/python2.7/dist-packages/gi/overrides/Gtk.py", line 744, in get_iter
    path = TreePath(path)
  File "/usr/lib/python2.7/dist-packages/gi/overrides/Gtk.py", line 1110, in __new__
    if len(path) == 0:
TypeError: object of type 'NoneType' has no len()

--------------------------------------------------------------------

RULES

    * Do not attempt to update the diff_view if the path is None.
    * Move the MockMethod helper to a common module so that it can be used
      by any test case
    * Update the DiffWidget to guard the calls to show(), subclasses the
      widget so that tests can run without rendering.

QA

    Using Precise:
    * Branch any project
    * bzr gdiff
    * See no changes
    * close the window
    * Verify there is no traceback in the console.