Run ls-files from the root of the directory

Summary:
One of our users ran into an issue where they were running arc diff in
a subdirectory and it so happened that in that directoy there wasn't any lfs
files so the repo was marked as $is_lfs == false. Lets allow arc to be run
from anywhere.

Test Plan: Internally this patch worked

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D21045