Pass the file contents instead of the file path to `phutil_json_decode`.

Summary: See D9681#22.

Test Plan: Created an empty `.git/arc/config` file... no exception was thrown.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9704