pack-objects: Depth can be negative
Remove leftover function declaration
protocol: basic support for multi_ack_detailed

This tells the server that we speak it, but we don't make use of its
extra information to determine if there's a better place to stop
negotiating.

In a somewhat-related change, reorder the capabilities so we ask for
them in the same order as git does.

Also take this opportunity to factor out a fairly-indented portion of
the negotiation logic.
merge: any non-zero return from the user is an error

This fixes #1703.
Merge pull request #1919 from libgit2/cmn/multi-ack-detailed

protocol: basic support for multi_ack_detailed