Feature request: Respect .gitignore'd files

[LordFckHelmchen]

Context

I really like the possibility to remove formatting considerations from my developers minds. Mdformat is a great tool to add for this endevour. My use case is running the mdformat linter in build/ci scripts where we don't have pre-commit available.

However, currently any *.md files are picked up - including any files in .gitignored folders like local virtual environments or Python caches. To assure that we pick up any new markdown files in the repository, we can not simply whitelist certain folders, but want to scan the whole repo instead. Ideally we also don't want to keep a separate list of ignored files, as there should be a single ground truth.

Proposal

• Add an option to respect a .gitignore file.
• If the option is set, the listed paths/files are ignored during discovery of the Markdown files.
• Consider how to find the .gitignore file. It should be on the same level as the config file/pyproject.toml file on repo root. But that might not always be the case (e.g. for mono-repos). Alternatively you could subprocess-call git ls-files, which will provide a list of files to process.
• This issues is related to #359

Tasks and updates

• [ ] Decide on discovery mechanism for .gitignore or on using git ls-files
• [ ] Implement filtering in run method
• [ ] Add tests for nested .gitignore files

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