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