Fix `arc browse path:line`

Summary: Ref T5781. This was broken in some earlier changes in T5781.

Test Plan: Ran `arc browse README`, `arc browse README:10`.

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5781

Differential Revision: https://secure.phabricator.com/D10346