File excluded even if explicitly passed via the command line when specified via --exclude

[ichard26]

Describe the bug

--exclude is supposed to only apply to the recursive discovery of files. Not files explicitly passed to Black through the command line. This is a regression since #1032 was merged.

To Reproduce

1. Create a directory for testing
2. Create and activate a virtual enviroment
3. Install Black on master using pip install git+git://github.com/psf/black
4. Create two empty files:
   4.1. pyproject.toml so Black doesn't find a different configuration file up your directory structure
   4.2. test.py - this will be our test file
5. Run black -v --exclude=test.py test.py
6. Install Black 19.10b0 using pip install black==19.10b0
7. Run black -v --exclude=test.py test.py
8. Observe that on master the file was excluded and on stable the file was formatted.

Expected behavior

Black formats test.py since it was explicitly passed through the command line and --exclude should not touch such files like it.

Environment

• Version: master (b59a524) and stable (19.10b0)
• OS and Python version: Ubuntu 18.04.04 LTS and Python 3.8.1

Does this bug also happen on master?

Yes and also N/A - This is a regression

Additional context

I haven't looked into this bug that much, but the problem seems to be related to how even files passed via the command line are given to the recursive gen_python_files function as single Path object lists in get_sources.

black/src/black/__init__.py

Lines 605 to 610 in b59a524

	elif p.is_file():
	sources.update(
	gen_python_files(
	[p], root, None, exclude_regexes, report, get_gitignore(root)
	)
	)