Add an option to `reuse lint` to only lint tracked files

[svenstaro]

I would like to request a feature to reuse lint (maybe --only-tracked?) that only checks files tracked by the VCS. The idea is that sometimes if there's temporary build output or ad-hoc experiments, those files should not be checked by reuse lint. I know that reuse lint tries to ignore files from .gitignore but sometimes with ad-hoc files, you don't want to explicitly add them to the .gitignore.