Fix misdetection of project root when --stdin-filename filepath is outside the current working directory

[n-borges]

Description

Fixes behaviour that prevented formatting for files outside the current working directory (#3207).

# before the fix
❯ cat ../script.py | python -m black --stdin-filename '../script.py' -
No Python files are present to be formatted. Nothing to do 😴

# after the fix 
❯ cat ../test.py | python -m black --stdin-filename '../test.py' -   
print("hello world")
All done! ✨ 🍰 ✨
1 file left unchanged.

Let me know what you think! Not sure about how elegant the solution is, but I would be happy to refactor it based on your suggestions.

Checklist - did you ...

• Add a CHANGELOG entry if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

[github-actions]

diff-shades reports zero changes comparing this PR (3daf8a7) to main (507234c).

---

What is this? | Workflow run | diff-shades documentation

[felix-hilden]

Oh, this seems to overlap with #3216 most unfortunately. Although it is quite a simple solution. Not sure which would be better, but I'd like to see a more concrete test related to the original issue if you're up for the race :P

[ichard26]

Hey, so #3216 was filed as an alternative fix for the same issue. I chose it over yours as it's probably more maintainable in the longterm by maintaining the invariant the project root contains the files to be formatted. Thanks for submitting a PR anyway, perhaps they got interested in fixing this issue too by your PR :)

Have a great day/night!

[n-borges]

Great, happy this issue has been solved!