hash-functions: fix the definition of hash trees

[multun]

No description provided.

[lvh]

I think there are two competing definitions out there, both are in use, they just disagree about which direction the arrows point.

[multun]

It's not just about the direction of the arrow, as I understand it, the current definition implies each node only references a single ancestor. I fact, I don't even think the patented merkle tree matches this definition, as each node hashes multiple ancestors (or children, depending on the direction of the arrows)

In practice, hash tree nodes often reference multiple nodes (git's directory nodes reference multiple files, git commits reference a tree object and the previous commit). I'd rather stick with the terminology of the hash tree wikipedia page, so people don't get confused if they look it up.