Silence a warning about unknown fields in 'getcommitmessage'

Summary: Similar to D8491, some of my new exceptions are a bit too aggressive. See IRC. This one's hitting an edit workflow with 'revisionID' onboard somehow.

Test Plan: Not entirely sure how to hit this, but it won't throw anymore.

Reviewers: btrahan, dctrwatson, zeeg

Reviewed By: zeeg

Subscribers: zeeg, aran, epriestley

Differential Revision: https://secure.phabricator.com/D8514