Teach the .git/config guard the skipFetchAll stamp

The integration harness normalizes away exactly the URL-keyed promisor+partialclonefilter pair git writes during filtered fetches; the new skipFetchAll/skipDefaultUpdate stamp made the section no longer match, tripping the guard in TestHTTPS_OutOfSyncCheckpointBranchRebases. Allow the two stamp keys in the section while still failing loudly when the core promisor pair is incomplete or any other key appears.