CHANGES:

• Add missing dependencies on Unix (@dra27, #573)
• Be able to choose the zlib compression level when we generate a PACK file (@dinosaure, #578)
• Fix spurious bug when we encode a patch into a PACK file (@dinosaure, #578)
• Add an accessor to get the hash ctx computed by the first-pass of a PACK file (@dinosaure, #584)
• Fix how we record the weight of Git objects into a PACK file (@dinosaure, #591)