fix(checkpoint): stamp phantom remotes left by failed filtered fetches

Git writes remote.<url>.promisor eagerly during connection setup, so a filtered fetch that later fails (e.g. a missing ref during resume) still leaves the URL-keyed phantom remote behind. The previous logic returned early on fetch error before stamping, and because it only stamped sections that did not exist beforehand, the entry then looked pre-existing on every later attempt and lingered unstamped forever.

Check section existence before and after the fetch and stamp when it newly appeared, regardless of fetch success. The post-fetch recheck also avoids inventing a section when the fetch died before git wrote anything.