빚기 전에 베이스를 골라

def choose_base(repo) -> str:
    """Decide what to build on, BEFORE building anything."""
    branch = current_branch(repo)
    run(["git", "fetch", "origin"], cwd=repo)      # ask first

    local = rev(repo, branch)
    remote = rev(repo, f"origin/{branch}", missing_ok=True)

    if remote is None or local == remote:
        return local                                # level

    if is_ancestor(repo, local, remote):            # BEHIND
        touched = run(["git", "diff", "--name-only", local, remote,
                       "--", *land_paths], cwd=repo).stdout.strip()
        if touched:
            raise SystemExit(
                "the remote moved THIS unit's paths while the claim "
                f"was held - reconcile by hand:\n{touched}")
        return remote            # build on the remote tip, not the
                                 # stale local one

    if is_ancestor(repo, remote, local):            # AHEAD
        return local             # keep the unpushed local commits

    raise SystemExit(               # DIVERGED
        "local and remote have both moved. no automated answer is "
        "correct here - merge or rebase depends on intent. "
        "reconcile by hand, then re-run.")


# Four branches, three of them decided by state alone.
# The refusal is the feature. Every increment of cleverness here
# moves a decision that needed a human into a code path nobody
# reads until it quietly does the wrong thing.