Run ls-files from the root of the directory
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

codelabs-ch/arcanist,phacility/arcanist,phacility/arcanist,codelabs-ch/arcanist,codelabs-ch/arcanist,phacility/arcanist,Khan/arcanist,Khan/arcanist,Khan/arcanist,phacility/arcanist,phacility/arcanist,codelabs-ch/arcanist,codelabs-ch/arcanist,Khan/arcanist