quizzes/git: Add 'git add / commit' question

Remove the default timeout, to give users who spawn `$EDITOR` via `git
commit` time to write and save their message.
question: Pass stdin through to the ScriptQuestion-invoked script

Use a temporary script to hold the answer script, to leave stdin open.
This allows users to use commands that need stdin (e.g. `git commit`,
which spawns an `$EDITOR`).
question: Add a trailing newline to ScriptQuestion scripts
quizzes/git: Add 'git checkout HEAD -- FILE' question