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.