compute_changes.py over-selects when a GHA workflow file is edited

# Bug report

### Bug description:

E.g. https://github.com/python/cpython/pull/144491/checks only edited `reusable-wasi.yml`, and yet most platforms were run.

### CPython versions tested on:

CPython main branch

### Operating systems tested on:

_No response_


### Linked PRs
* gh-144518