Check only files tracked by git

[kowal]

For Rubocop I often get warnings about offences in files, which I create only locally (some scripts) - they are not tracked by git however Rubocop (and possibly other tools) complain about them.

Maybe it would be possible to configure all tools to ignore files not tracked by git.

[gajewsky]

Also it would be really nice to allow to ignore specific directories/files from all checks, even if they are tracked by git. For now we can only ignore rspec_files_structure and final_blank_line.

[kowal]

Issue was reported to Rubocop long time ago (rubocop/rubocop#595).