Add a smoke test

We need to have at least one test for `src/bin` (run on real files).

Idea: git checkout a repo/commit, run the command, snapshot the `git diff` on the files changed?

Before release I try to run on a real folder anyway but this would be nice.

Start in src/bin and identify how the command is run against real files. Explore the proposed flow of checking out a repository or commit, running the command, and snapshotting git diff output; done means at least one repeatable smoke test covers a real-file upgrade scenario.

Written by the indexing model from the issue text.