Blockchain is just a special case of Merkle trees, there isn't anything original about them other than that Bitcoin served as a marketing engine for the term blockchain because some people made a ton of money with it.

https://en.wikipedia.org/wiki/Merkle_tree

No block chains are different than Merkle trees entirely. Block chains include previous hashes in each block, whereas Merkle trees, as the name implies are trees of hashes. In Merkle trees blocks do not include the previous block's hash.

In git each "block" includes the previous "block"(s)' hash. Is it a blockchain or a hash tree?

I would say that in practice what differentiates a blockchain from other applications of hash trees is a mechanism for consensus, not whether the blocks being formed into a trees conceptually represent time or not.

Git is a block chain. Block chains require a previous block's hash to denote sequence. The hash for any block of data can be appended to Merkle tree, even duplicate blocks.

In a block chain it is easy to find history because the link to the history is included. Merkle trees require n-1 additional hashes.

Disagree. There are not formal definitions for these terms, but I don't think your definition here is what most people would think of as a blockchain.

Looking at the wikipedia article I can see where one might be confused.

    A blockchain is a growing list of records,
    called blocks, that are linked together using 
    cryptography. Each block contains a cryptographic
    hash of the previous block, a timestamp, and
    transaction data (generally represented as a
    Merkle tree).

The Merkle tree referenced here is with respect to the organization of the transaction data contained within a block, not the blockchain itself.

Merkle trees are used in various ways within cryptography in general and cryptocurrencies specifically, but blockchains and Merkle trees are distinct data structures with different uses. The colloquial use of "blockchain" has perhaps made the word somewhat ambiguous in some contexts but not in the context of cryptographic data structures, and Merkle trees are in fact formally defined.

You can visualize how immudb Merkle tree grows as you insert data on https://play.codenotary.com

> there isn't anything original about them

Cool, so let's say we're using this to track financial transactions.

Server A has an Immudb at some state x and server B has an Immudb at some state y. Which one is correct, how do I decide?

Rekor is just that. It's a merkle tree implementation (with extras such as timestamping)

https://github.com/sigstore/rekor

And with git being the most superior blockchain of them all.

Except it has no way to achieve consensus automatically. That's left as an exercise to the reader.

It's only the actually-useful bits of a "blockchain" without the planet-cooking proof-of-waste consensus algorithm brute-forcing sha256 over and over again.

And also without the useful "automatically achieve consensus between untrusted parties" bit.