code: async function getBranchPr(branchName) {
    logger.debug(`getBranchPr(${branchName})`);
    if (!(await branchExists(branchName))) {
      return null;
    }
    const urlString = `projects/${
      config.repository
    }/merge_requests?state=opened&per_page=100`;
    const res = await get(urlString, { paginate: true });
    logger.debug(`Got res with ${res.body.length} results`);
    let pr = null;
    res.body.forEach(result => {
      if (result.source_branch === branchName) {
        pr = result;
      }
    });
    if (!pr) {
      return null;
    }
    return getPr(pr.iid);
  }
docs: Returns the Pull Request for a branch. Null if not exists.