make getFileSize file system independent

Follow up from #113.

For the git comparator numbers for measuring the checkout folder size are different between CircleCI, GH actions and local Mac in the unit tests (`local branch` for example) even though the contents of the folders are identical. This might be because of different Linux versions and/or file systems.

Compare
- https://app.circleci.com/pipelines/github/adobe/sizewatcher/319/workflows/25d54edb-292d-452a-af16-cc4dcf226fdf/jobs/315
- https://github.com/adobe/sizewatcher/actions/runs/14213116331/job/39823823685?pr=113

Start by locating getFileSize and the unit tests that measure the checkout folder, including the “local branch” case. Compare the results from the linked CircleCI and GitHub Actions runs with local output; done means identical folder contents produce consistent sizes across these environments.

Written by the indexing model from the issue text.