exclude directories handled differently for flake8 diff

[tnh8]

Please describe how you installed Flake8

$ pip install flake8

Please provide the exact, unmodified output of flake8 --bug-report

C:\>flake8 --bug-report
{
  "dependencies": [],
  "platform": {
    "python_implementation": "CPython",
    "python_version": "3.7.9",
    "system": "Windows"
  },
  "plugins": [
    {
      "is_local": false,
      "plugin": "mccabe",
      "version": "0.6.1"
    },
    {
      "is_local": false,
      "plugin": "pycodestyle",
      "version": "2.8.0"
    },
    {
      "is_local": false,
      "plugin": "pyflakes",
      "version": "2.4.0"
    }
  ],
  "version": "4.0.1"
}

Please describe the problem or feature
Running two flake8 jobs, a full scan and a diff check, only one of them respects the directories in exclude sections when written as described in the documentation examples I found 1 2.

If this is a bug report, please explain with examples (and example code) what you expected to happen and what actually happened.

I'm using the following configuration:

setup.cfg:

[flake8]
statistics = True
show-source

exclude =
    thirdParty

Then add some error in some file placed in directory named thirdParty.

Running the following job does not produce any findings:

flake8 --append-config=setup.cfg .

But running this command will find the issue:

git diff -- '*.py' | flake8 --diff  --append-config=setup.cfg -

my output:

$ git diff -- '*.py' | flake8 --diff --append-config=setup.cfg -
python/thirdParty/merge.py:95:21: W291 trailing whitespace
1     W291 trailing whitespace

Workaround:
changing the exclude section to:

exclude =
    */thirdParty/*

makes also the diff example run without any findings.

[sigmavirus24]

--diff is equivalent to telling flake8 lint these files and only report errors on changed lines. --exclude is difficult here. I'm also fairly certain that --diff should disappear

[asottile]

in #1389 we're planning to significantly change or remove --diff