Clarify git check-ref-format --branch behavior in/out of git repo

[Copilot]

Answered review question about whether git check-ref-format --branch behaves identically inside vs outside a git repository.

Behavior Difference

The --branch flag expands "previous checkout syntax" (@{-1}, @{-2}, etc.) to actual branch names only when run inside a git repository. Outside a repo, it treats these as invalid branch names.

Current Implementation

The is_valid_branch_name() function correctly uses cwd=empty_dir to ensure consistent validation independent of repository checkout history:

def is_valid_branch_name(branch: str) -> bool:
    with tempfile.TemporaryDirectory() as empty_dir:
        command = ["git", "check-ref-format", "--branch", branch]
        result = subprocess.run(command, cwd=empty_dir, capture_output=True, text=True)
        return result.returncode == 0

This validates the branch name string itself, not what it might expand to in a specific repository context.

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