Symfony version(s) affected: >=4.3

Description

When you want to ignore everything in a certain directory, except some specific files,
these are the .gitignore rules that can be used:

/example/**
!/example/example.txt
!/example/packages/
!/example/packages/example.yaml

You might think /example/packages/ will be unignored by this, but that is not the case according to the .gitignore standard.
With this setup, any files in /example/packages/ besides /example/packages/example.yaml will be ignored. But the Gitignore::toRegex does not match this logic properly.

See also: https://git-scm.com/docs/gitignore and https://stackoverflow.com/a/61556103/3265437

How to reproduce
https://github.com/Jeroeny/reproduce/tree/gitign

Possible Solution
The regex building logic would have to be adjusted. I've looked into this but wasn't able to fix it yet.

Additional context
See the reproducer.