System.err.println("target_branch_name must not be empty");
            System.exit(2);
        }

        String targetRef = "refs/remotes/origin/" + targetBranch;

        if (!refExists(targetRef)) {
            System.err.println("Target ref " + targetRef + " not present locally; fetching origin/" + targetBranch + "...");
            run("git", "fetch", "origin", targetBranch);
        }