Blog post: https://miguelmota.com/blog/checkpoint-git-commit-objects-to-ethereum-and-verify-with-merkle-proofs/

Commits.json

{
  "contractName": "Commits",
  "abi": [
    {
      "constant": true,
      "inputs": [
        {
          "name": "",
          "type": "bytes20"
        }
      ],
      "name": "checkpoints",
      "outputs": [
        {
          "name": "",
          "type": "uint256"
        }
      ],
      "payable": false,
      "stateMutability": "view",
      "type": "function"
    },
    {
      "anonymous": false,
      "inputs": [
        {
          "indexed": true,
          "name": "sender",
          "type": "address"
        },
        {
          "indexed": true,
          "name": "commit",
          "type": "bytes20"
        }
      ],
      "name": "Checkpointed",
      "type": "event"
    },
    {
      "constant": false,
      "inputs": [
        {
          "components": [
            {
              "name": "tree",
              "type": "string"
            },
            {
              "name": "parents",
              "type": "string[]"
            },
            {
              "name": "author",
              "type": "string"
            },
            {
              "name": "authorDate",
              "type": "uint256"
            },
            {
              "name": "authorDateTzOffset",
              "type": "string"
            },
            {
              "name": "committer",
              "type": "string"
            },
            {
              "name": "commitDate",
              "type": "uint256"
            },
            {
              "name": "commitDateTzOffset",
              "type": "string"
            },
            {
              "name": "message",
              "type": "string"
            },
            {
              "name": "signature",
              "type": "string"
            }
          ],
          "name": "_commit",
          "type": "tuple"
        }
      ],
      "name": "checkpoint",
      "outputs": [
        {
          "name": "commitHash",
          "type": "bytes20"
        }
      ],
      "payable": false,
      "stateMutability": "nonpayable",
      "type": "function"
    },
    {
      "constant": true,
      "inputs": [
        {
          "name": "commit",
          "type": "bytes20"
        }
      ],
      "name": "checkpointed",
      "outputs": [
        {
          "name": "",
          "type": "bool"
        }
      ],
      "payable": false,
      "stateMutability": "view",
      "type": "function"
    },
    {
      "constant": true,
      "inputs": [
        {
          "name": "commit",
          "type": "bytes20"
        },
        {
          "name": "root",
          "type": "bytes20"
        },
        {
          "name": "leaf",
          "type": "bytes20"
        },
        {
          "name": "proof",
          "type": "bytes20[]"
        }
      ],
      "name": "checkpointVerify",
      "outputs": [
        {
          "name": "",
          "type": "bool"
        }
      ],
      "payable": false,
      "stateMutability": "view",
      "type": "function"
    },
    {
      "constant": true,
      "inputs": [
        {
          "name": "root",
          "type": "bytes20"
        },
        {
          "name": "leaf",
          "type": "bytes20"
        },
        {
          "name": "proof",
          "type": "bytes20[]"
        }
      ],
      "name": "verify",
      "outputs": [
        {
          "name": "",
          "type": "bool"
        }
      ],
      "payable": false,
      "stateMutability": "pure",
      "type": "function"
    }
  ],
  "compiler": {
    "name": "solc",
    "version": "0.5.8+commit.23d335f2.Emscripten.clang"
  },
  "networks": {
    "42": {
      "events": {},
      "links": {},
      "address": "0xBA7619DFA824A168fDD76a77b8236f83528d015C",
      "transactionHash": "0x52554e323a127444d04e84b5f3712f2d168d2c325fc288b7ea4e6da2ebb725c2"
    }
  },
  "schemaVersion": "3.0.11",
  "updatedAt": "2019-07-28T10:46:05.506Z",
  "devdoc": {
    "methods": {}
  },
  "userdoc": {
    "methods": {}
  }
}

index.js

const assert = require('assert')
const fs = require('fs')
const path = require('path')
const { execSync } = require('child_process')
const Web3 = require('web3')
const PrivateKeyProvider = require('truffle-privatekey-provider')
const parseCommit = require('git-parse-commit')

const contractJSON = JSON.parse(fs.readFileSync(path.resolve(__dirname, './Commits.json')))
const { abi } = contractJSON
const networkId = 42 // kovan
const { address: contractAddress } = contractJSON.networks[networkId]

const privateKey = execSync('git config ethereumcheckpoint.privatekey').toString().trim()
const providerUri = execSync('git config ethereumcheckpoint.provideruri').toString().trim()
const provider = new PrivateKeyProvider(privateKey, providerUri)
const web3 = new Web3(provider)

;(async () => {
  const { address: sender } = web3.eth.accounts.privateKeyToAccount(`0x${privateKey}`)

  const commit = execSync('git cat-file -p HEAD').toString().trim()
  const commitHash = execSync('git rev-parse HEAD').toString().trim()
  const tag = execSync('git describe --tags `git rev-list --tags --max-count=1`').toString().trim()
  const tagCommit = execSync(`git rev-list -n 1 "${tag}"`).toString().trim()

  if (tagCommit !== commitHash) {
    console.log('Tag not found for commit, skipping checkpoint.')
    process.exit(0)
  }

  console.log(`Tag ${tag} found, checkpointing commit ${commitHash}`)

  const {
    tree,
    parents,
    author: {
      name: authorName,
      email: authorEmail,
      timestamp: authorDate,
      timezone: authorDateTzOffset
    },
    committer: {
      name: committerName,
      email: committerEmail,
      timestamp: commitDate,
      timezone: commitDateTzOffset
    },
    pgp,
    title,
    description
  } = parseCommit(`${commitHash}\n${commit}`)

  const author = `${authorName} <${authorEmail}>`
  const committer = `${committerName} <${committerEmail}>`
  const message = `${title}${description}
`
  let signature = ''
  if (pgp) {
  // NOTE: newlines are necessary here
  signature = `-----BEGIN PGP SIGNATURE-----

${pgp}
-----END PGP SIGNATURE-----`.split('\n').join('\n ') + '\n'
  }


  const data = {
    tree,
    parents,
    author,
    authorDate,
    authorDateTzOffset,
    committer,
    commitDate,
    commitDateTzOffset,
    message,
    signature
  }

  const contract = new web3.eth.Contract(abi, contractAddress)

  try {
    console.log('Checkpointing commit to Ethereum...')
    const {
      status,
      transactionHash
    } = await contract.methods.checkpoint(data).send({
      from: sender
    })

    console.log(`Transaction hash: ${transactionHash}`)
    assert.ok(status)

    const _commitDate = await contract.methods.checkpoints(`0x${commitHash}`).call()
    assert.equal(_commitDate.toString(), commitDate.toString())
    console.log('Successfully checkpointed commit to Ethereum.')

    process.exit(0)
  } catch(err) {
    console.error(err.message)
  }
})()

package.json

{
  "name": "ethereum-checkpoint-git-commit",
  "version": "0.0.1",
  "dependencies": {
    "git-parse-commit": "^1.0.0",
    "web3": "^1.2.0",
    "truffle-privatekey-provider": "^1.3.0"
  }
}