Exclude URL-keyed promisor entries from git fetch --all

Every filtered checkpoint fetch from a URL leaves a remote.<url> promisor config section behind (git needs it for lazy object fetches), and git treats each one as a fetchable remote: git fetch --all and git remote update dial every checkpoint URL ever fetched from, including dead mirror hosts and SSH github URLs. Stamp skipFetchAll/skipDefaultUpdate on the entry right after a filtered URL fetch so bulk fetches skip it while lazy fetches keep working. Gated on git having actually written the promisor entry, so no section is invented for unfiltered fetches.