REQ-GIT-082: Thin-pack REF_DELTA bases resolve from packfiles, not only from loose objects

git push sends a thin pack by default, whose REF_DELTAs are cut against objects the server advertised and therefore omitted from the pack. ReceivePack must supply those bases from wherever the repository actually stores them — a packfile as readily as a loose file. A repository whose objects have been packed (the steady state after any repack, and what Maintenance.repack/1 deliberately produces) must accept a thin push. A REF_DELTA naming a base the repository genuinely does not have must still be rejected.