Depends on rename/copy detection.

Scope

• Walk.log_follow(repo, path, start_sha, opts) yields commits that touched path, following renames backward.
• Each yielded entry includes the path-at-that-commit and the rename chain if any.

Acceptance

• Matches git log --follow -- <path> order and set on a fixture with a file renamed twice.
• Stops walking when the file originates (add with no rename source).