Load relationships for revision

Summary:
In the commit message parser worker, it tries to get relationships for a
revision without first loading them. (D3444 introduced the change.) This
is where the get happens, so I'm adding the load here - maybe it should be
added in the CommitMessageParserWorker instead?

Test Plan: Made this change, and my taskmaster daemons stopped failing.

Reviewers: epriestley, vrana

Reviewed By: vrana

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3448