Fix gtk dialogs popping up and asking for input during selftest.
* tests/test_commit.py: (TestCommitDialog_Commit._set_question_yes, TestCommitDialog_Commit._set_question_no): Accept keyword arguments (but ignore them). (TestCommitDialog_Commit.test_commit_empty_message): Rename the test to avoid confusion with test_commit_no_messages.
* commit.py: (CommitDialog.__init__): Restore the ability to override the dialog used for automated tests. (CommitDialog._do_commit, CommitDialog._do_commit, CommitDialog._do_commit): Use the dialog class defined at construction time.