The _init_repo helper calls Repo.init(repo, b="master") at line 34, which passes git init -b master under the hood. The -b flag to specify the initial branch name was introduced in Git 2.28; on older git versions the command fails with error: unknown switch 'b', causing every test in the suite to error out during setUp.

Proposed solution: Make the -b usage conditional on the git version using Git().version_info >= (2, 28) (GitPython exposes this as a plain int tuple), falling back to r.git.symbolic_ref("HEAD", "refs/heads/master") on older git.