This exists purely to verify the behavior of --only-changed with --storybook-build-dir when pointed at a Storybook that was built in a subdirectory (such as this one).
In this scenario, paths in preview-stats.json will use subdir as their root directory (./), which means they'll be misaligned with the git root directory. For example, git may report ./subdir/One.js as changed, but since this file is reported as ./One.js in the webpack stats, it will not find any dependent stories for that file. Passing --storybook-root-dir subdir will resolve that.