対象のブランチの派生元commitかmerge commitのhashを表示する

hoge.js

const gmk = require('git-miru-kun');

(async () => {
  const gitDir = await gmk.readGitDir('.git');
  
  const parentHashDict = {};
  
  const targetBranch = await gitDir.readHead();
  const { branchName } = targetBranch;
  const commits = (await gitDir.readBranches())
    .filter(b => b.branchName !== branchName)
    .map(b => b.commit);
  
  let baseCommitHash;
  let x = 1;
  
  while (!baseCommitHash) {
    do {
      for (let i = commits.length - 1; i >= 0; i--) {
        let commit = commits[i];
        for (let j = 0; j < 1000; j++) {
          const { parentHash } = commit;
          if (!commit.hasParent() || parentHashDict[parentHash]) {
            commits.splice(i, 1);
            break;
          }
          if (commit.isMergeCommit()) {
            commits.push(await commit.readMergedParent());
          }
          parentHashDict[parentHash] = true;
          try {
            commit = await commit.readParent();
          } catch (e) {
            commits.splice(i, 1);
            break;
          }
        }
      }
    } while (commits.length > 1);
    
    let { commit } = targetBranch;
  
    for (let i = 0, n = x * 16; i < n; i++) {
      console.log(commit.hash, commit.isMergeCommit());
      if (commit.isMergeCommit()) {
        baseCommitHash = commit.hash;
        break;
      }
      if (parentHashDict[commit.parentHash]) {
        baseCommitHash = commit.parentHash;
        break;
      }
      commit = await commit.readParent();
    }
    
    x++;
  }
  
  console.log('base commit hash is ', baseCommitHash);
})();

text.md

.gitディレクトリ内のファイルからcommitを探索するやつ https://github.com/ukyo/git-miru-kun

$ npm i -g https://github.com/ukyo/git-miru-kun
$ cd path/to/repos
$ node hoge.js

まぁ、これ100%確実に発見できるわけではなくて、現実的には平気だろっていうレベルのやつです。