Same root cause as 16cf9edf — checkout-git emits mainRepoRoot directly,
so the helper must initialize it to null or downstream isWorktree
derivation flips to true on undefined !== null.
Same root cause as 16cf9edf — checkout-git emits mainRepoRoot directly,
so the helper must initialize it to null or downstream isWorktree
derivation flips to true on undefined !== null.