Only lint files in the working copy with `arc lint --everything`.

Summary: Fixes T5442. `arc lint --everything` currently uses a `FileFinder` to discover paths for linting. A consequence of this is that files that are ignored or otherwise excluded from version control are linted.

Test Plan: Ran `arc lint --everything` in rPHU and noticed that I wasn't prompted to add a trailing newline to `src/.phutil_module_cache`.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5442

Differential Revision: https://secure.phabricator.com/D9674