test: make tip-read regression test exercise a real fetch

PR review (Copilot): the plain git clone <bareDir> fetched all branches via the local hardlink optimization, so refs/remotes/origin/entire/checkpoints/v1 and its history were already present before FetchMetadataTreeOnly ran — weakening the assertions. Clone single-branch main via file:// so the metadata branch is absent until the tip-read fetches it; the old --depth=1 behavior now visibly shallows the repo and trips the assertion.