Code restructuring and a small fix to transport protocol v2 during
fetching.
* jt/fetch-pack-negotiator:
fetch-pack: introduce negotiator API
fetch-pack: move common check and marking together
fetch-pack: make negotiation-related vars local
fetch-pack: use ref adv. to prune "have" sent
fetch-pack: directly end negotiation if ACK ready
fetch-pack: clear marks before re-marking
fetch-pack: split up everything_local()