Parse PR numbers from commit messages. At GitHub those have the format:

        `here is the message (#1234)`

    being `1234` the PR number.

def parse_pr_numbers(git_log_lines):
    """
    Parse PR numbers from commit messages. At GitHub those have the format:

        `here is the message (#1234)`

    being `1234` the PR number.
    """
    prs = []
    for line in git_log_lines:
        pr_number = parse_pr_number(line)
        if pr_number:
            prs.append(pr_number)
    return prs