Negate commit add-unstaged-files question

Summary:
The add-unstaged-files question arc asks differs in positivity in
different workflows. `arc diff` asks "Do you want to add the files
before continuing", while `arc land` asks "Do you want to continue
without adding?".

This normalizes the questions to have the same "positivity".
Specifically, it negates the `arc land` workflow, because that seemed a
little more isolated.

Test Plan:
Touch a file, run both `arc diff --preview` and `arc land --preview`