Merge branch 'jt/v2-fetch-nego-fix'

The upload-pack protocol v2 gave up too early before finding a common ancestor, resulting in a wasteful fetch from a fork of a project. This has been corrected to match the behaviour of v0 protocol. * jt/v2-fetch-nego-fix: fetch-pack: in protocol v2, reset in_vain upon ACK fetch-pack: in protocol v2, in_vain only after ACK fetch-pack: return enum from process_acks()