Bad error message when run in empty repo

```console
> minion run --containerfile ./Containerfile -m "Do something"
Do something

Working on the task.

thread 'main' panicked at src/cli/mod.rs:106:22:
Failed to run task: reference 'refs/heads/main' not found; class=Reference (4); code=UnbornBranch (-9)
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```