Add branch selection option to rendergit

**Description:**
Currently, `rendergit` always clones the default branch (`main`/`master`) of a repository. It would be very useful to allow users to specify which branch to render.

**Proposed solution:**
- Add a CLI flag (e.g. `--branch `) to `rendergit`.
- When provided, `rendergit` should clone the specified branch instead of the default one.
- If no branch is specified, fallback to the default branch as it works today.

**Example usage:**
```bash
rendergit https://github.com/karpathy/nanoGPT --branch develop
```

Start at the rendergit CLI argument parsing and the code that clones the repository. Run the current command with a repository URL, then add the branch option while preserving the existing default-branch behavior when it is omitted. Done means the specified branch is rendered and the no-flag case still works.

Written by the indexing model from the issue text.