Hashes are binary values internally, so "z" (or it’s ASCII byte value) already exists as a prefix in SHA-1 hashes (any hash that begins with 7a). The real problem is that existing code assumes that hashes are always exactly 20 bytes in size, and any 20-byte value is a valid SHA-1 hash. There is no way to indicate a non-SHA-1 hash in the binary Git data formats and internal representations processed by existing code. The task is to change those formats and representations, and the related code, to enable such an indication.
