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

Define _last_selected_file in init so that async signal callbacks can use it.

I occasionally see the following traceback when committing use bzr-gtk
r769 on precise:

  File "/home/curtis/.bazaar/plugins/gtk/commit.py", line 579, in _on_treeview_files_cursor_changed
    self._update_per_file_info(selection)
  File "/home/curtis/.bazaar/plugins/gtk/commit.py", line 618, in _update_per_file_info
    self._save_current_file_message()
  File "/home/curtis/.bazaar/plugins/gtk/commit.py", line 605, in _save_current_file_message
    if self._last_selected_file is None:
AttributeError: 'CommitDialog' object has no attribute '_last_selected_file'

The commit is successful. Looking at the order of events,
_construct_file_list() connects _on_treeview_files_cursor_changed() to
the cursor-changed signal. _on_treeview_files_cursor_changed() presumes
_last_selected_file() was created, but it will not be created until
_fill_in_per_file_info() is called (which is two lines before
_construct_file_list())

The cursor-changed signal may have been called after _fill_in_per_file_info(),
but it is often called before with Gtk3.

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

RULES

    * Ensure that CommitDialog._last_selected_file is defined before
      any signal callback is connected.

QA

While I do know the steps that I see the error, I do not always see it. Maybe
the issue only happens when there is a large tree of changes?
    * Run gcommit under precice
    * Type a few characters
    * Cancel the action
    * Save the message
    * Verify there is no AttributeError on strout