error when there is no shared history

```
thread 'main' panicked at 'no ahead behind stats found for = Some(RemoteBranch { remote_branch: "upstream/master", remote_branch_name: "master", remote_name: "upstream" })', src/models.rs:252:17
note: Run with `RUST_BACKTRACE=1` for a backtrace.
```

p-g-p should likely just not print anything in case such a thing happens