ui.cli: Fix single-line answer extraction

`var.get(key, default)` only works for dicts.  I don't think there is
an equivalent that works for lists.
quizzes/git: Add 'git help config' question
ui.cli: Fix ._set_ps1 when .question is None

Otherwise QuestionCommandLine will crash if its launched with an empty
stack (not that this should happen, but we should still avoid the
crash).
ui.cli: Quit after the last question

Oops, should have tested that when I transitioned to Cmd ;).
quizzes/git: Add 'gitignore' question