Drive revision update from Conduit via custom fields

Summary:
When we create or update a revision, we use a parsed commit message dictionary
to edit its fields. Drive consumption of the dictionary through custom fields
instead of hardcoding.

This requires adding some fields which don't really do anything right now to
cover fields which appear only in the commit message.

Test Plan: "arc diff"'d this revision against localhost, "arc diff"'d again to
update.
Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, jungejason
Differential Revision: 811