[aboodman]

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.

[joshuak]

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.

[aboodman]

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.

[joshuak]

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.