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

[brettcannon]

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-144492: Fix process_changed_files outputs for reusable-{macos, wasi}.yml #144518
• [3.13] gh-144492: Fix process_changed_files outputs for reusable-{macos, wasi}.yml (GH-144518) #144634
• [3.14] gh-144492: Fix process_changed_files outputs for reusable-{macos, wasi}.yml (GH-144518) #144635