BasePackFetchConnection: support negotiationTip feature
By default, Git will report, to the server, commits reachable from all local refs to find common commits in an attempt to reduce the size of the to-be-received packfile. If specified with negotiation tip, Git will only report commits reachable from the given tips. This is useful to speed up fetches when the user knows which local ref is likely to have commits in common with the upstream ref being fetched.
When negotation-tip is on, use the wanted refs instead of all refs as source of the "have" list to send.
This is controlled by the `fetch.usenegotationtip` flag, false by default. This works only for programmatic fetches and there is no support for it yet in the CLI.