Summary:
Git (the world's hardest revision control system) allows you to change
output formats by accident and/or without your direct knowledge. Protect users
from themselves.
Test Plan:
Changed "pretty" in [format] to "format:quack" so every log just
outputs the word "quack". Ran "arc diff" successfully.
Reviewed By: aran
Reviewers: aran
CC: epriestley, aran
Differential Revision: 56