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