Updating the Git protocol for SHA-256

Or add the prefix for all but sha-1. A first char of [0-9a-f] would mean sha-1 and prefix must not be removed. A prefix g would be sha-256, and so on. That's bot very different from MultiHash though. The pain is the complexity of the sha-1 exception which is not that awful for full hashes (you can look at the length, as all the other will have a prefix there's no risk of collision). Shortened hashes added a layer of mess to the problem ...