Implement commit message saving without modifying bzrlib.

* tests/test_commit.py:
(TestSavedCommitMessages.setUp): Install the post_uncommit hook
for all relevant tests.
(TestUncommitHook.setUp): Use explicit rev-ids to ease debugging.

* commit.py: 
Fix imports. Integrate SavedCommitMessagesManager so that we don't
need to modify bzrlib anymore.
(CommitDialog.__init__, CommitDialog._fill_in_files,
CommitDialog._construct_global_message, CommitDialog._do_cancel,
CommitDialog._do_commit): Stop testing can_save_commit_messages,
SavedCommitMessagesManager is always available now.
(SavedCommitMessagesManager): Borrowed from Anne Mohsen's patch.
(save_commit_messages): Implement the post_uncommit hook.

* __init__.py:
Install a lazy hook.