update-bookreader.js

const packageJson = require('./package.json');
const shrinkwrapJson = require('./npm-shrinkwrap.json');
const fetch = require('node-fetch');
const fs = require('fs');
const { execSync } = require('child_process');
const GH_API_ROOT = 'https://api.github.com';
const REPO = "internetarchive/bookreader";

async function main() {
    // Make sure branching off of latest master
    execSync(`git checkout master`, {stdio: 'inherit'});
    execSync(`git pull origin master`, {stdio: 'inherit'});

    // Fetch the latest release from GitHub
    const latestRelease = await fetch(`${GH_API_ROOT}/repos/${REPO}/releases/latest`).then(r => r.json());
    const newTag = latestRelease.tag_name;
    const [ref] = await fetch(`${GH_API_ROOT}/repos/${REPO}/git/matching-refs/tags/${newTag}`).then(r => r.json());
    const newSHA = ref.object.sha;

    // Update package.json
    packageJson.dependencies.bookreader = `git://github.com/${REPO}.git#${newTag}`;
    fs.writeFileSync('package.json', JSON.stringify(packageJson, null, "  ") + '\n');

    // Update shrinkwrap
    shrinkwrapJson.dependencies.bookreader.version = `git://github.com/${REPO}.git#${newSHA}`;
    shrinkwrapJson.dependencies.bookreader.from = `git://github.com/${REPO}.git#${newTag}`;
    fs.writeFileSync('npm-shrinkwrap.json', JSON.stringify(shrinkwrapJson, null, "  ") + '\n');

    // Bail if no changes
    const diff = execSync('git diff npm-shrinkwrap.json package.json').toString();
    if (!diff) {
        console.log("Everything already up-to-date");
        return;
    }

    // Create/push branch
    execSync(`git checkout -b br-${newTag}`, {stdio: 'inherit'});
    execSync(`git add npm-shrinkwrap.json package.json`, {stdio: 'inherit'});
    execSync(`git commit -m "Update BookReader to ${newTag}"`, {stdio: 'inherit'});
    execSync(`git push origin HEAD`, {stdio: 'inherit'});

    // Create MR
    const newMR = await fetch(`https://git.archive.org/api/v4/projects/58/merge_request`, {
        method: 'POST',
        headers: {
            'PRIVATE-TOKEN': process.env['GITLAB_TOKEN'],
            'Content-type': 'application/json',
        },
        body: JSON.stringify({
            source_branch: `br-${newTag}`,
            target_branch: 'master',
            title: `Update BookReader to ${newTag}`,
            description: `See changelog: ${latestRelease.html_url}`,
            labels: 'team-ux',
            remove_source_branch: true,
        }),
    }).then(r => r.json());

    // post link to MR on Slack
    await fetch(process.env['SLACK_HOOK'], {
        method: 'POST',
        headers: {
            'Content-type': 'application/json',
        },
        body: JSON.stringify({
            text: `Creating a new MR for BookReader ${newTag} at ${newMR.web_url}`
        }),
    });
}

if (execSync('git status --untracked-files=no --porcelain').toString()) {
    throw "Uncommited changes!";
}

main();

ooh, thanks for sharing the stdio inherit - TIL! :)
eg:

execSync(git checkout -b br-${newTag}, {stdio: 'inherit'});