Bugfix black .gitignore usage

[jedie]

Black didn't use the .gitignore file, because gen_python_files() will be called with gitignore_dict=None and None will disable the .gitignore usage.

see: #323 (comment)

Fixes #490.

[MatthijsBurgh]

I think this change should indeed change the behaviour, see https://github.com/psf/black/blob/35ce37ded7bd8fdd3950af19e7c11f311ee7b8d8/src/black/files.py#L361-L370

[akaihola]

This is great, thanks @jedie and @MatthijsBurgh.

Three questions come to mind:

• Might this change break something if using an old version of Black? If so, the minimum required Black version might need to be bumped. Which Black version does psf/black@35ce47e affect?
• How to write a unit or integration test for this?
• Why are all GitHub checks "Expected - Waiting for status to be reported" after @jedie submitted the patch?

[MatthijsBurgh]

I checked the blame, I think the the minimal required version of black is 21.5b1. Currently it is on 21.7b1, so fine.

[akaihola]

I wrote three unit tests, matching the three different versions of gen_python_files in supported Black versions.

Actually {} is a valid value only for the most modern versions which use the gitignore_dict argument. The older versions with a gitignore argument must still get None as the value instead. I fixed that too.

[akaihola]

It's ok for the future test to fail. Updating Black to 24.2 and beyond is now prevented for Darker 1.7.x, and version 1.8.0 will fix this compatibility issue and allow newer Black versions again.