CI failure when an unrelated PR has added a map

I'm looking at the result of a Wesnoth CI build, but AFAICS the files are the same in this repo.
https://github.com/wesnoth/wesnoth/actions/runs/7688512844/job/20949816292?pr=8281

```
Check out data/test/maps/4p_separate_castles.map from 062400441cdfbaf62f74736e919e875ccad7fcd4...
fatal: path 'data/test/maps/4p_separate_castles.map' exists on disk, but not in '062400441cdfbaf62f74736e919e875ccad7fcd4'
Error: Process completed with exit code 128.
```

That map isn't touched by https://github.com/wesnoth/wesnoth/pull/8281 . Some unrelated PR has added a map, but because the current PR's branch is based on an older version of master, the lack of that map file is being shown as a CI failure.