Follow-up to a3ec2e55c addressing code-review feedback:
- Restrict the existence check to local heads via
`git show-ref --verify --quiet refs/heads/<name>` so a stray tag or
remote ref with the same name cannot satisfy the check and lead the
subsequent `git checkout` to a detached HEAD on the wrong object.
- Use `git branch -M` (force) instead of `-m` for the rename path so
case-only renames succeed on case-insensitive filesystems (macOS,
default Windows).
- Document the FN-5456 invariant on the function's doc comment.
- Add a real-git regression suite covering the three paths the fix
governs: no creation when expected ref is missing, switch-only when
it exists, and rejection of same-named tags.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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