Support force-added files in a Git repository

Consider the following situation:
1. A user adds a file into `.gitignore`.
2. A user then adds the same file via `git add -f myfile.txt`.
3. We then expect our REUSE tooling to process this file. But it actually won't.

(Do we expect this? Need to check with the REUSE spec and actual tool behavior.)

I assume this should be fixed and we should process and not ignore such files.

See the code near this line: https://github.com/ForNeVeR/dotnet-reuse/blob/be3dd1818b0ea88685dae07cd847eab31fe6da46/FVNever.Reuse/ReuseDirectory.cs#L59

Also, look for the number of this issue in the project code base.