Ignore untracked files in lint repository cache key

Summary: Some people have 2GB+ untracked files in repo which significantly slows down this or even crashes it.

Test Plan: Added a debug output here and linted repo with untracked path.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, arudolph

Differential Revision: https://secure.phabricator.com/D4713