Allow system agents in commit message object lists

Summary:
When system agent adds a comment then he is added to CC.
When I amend and update then I get message "Commit message references nonexistent ..."

Test Plan: Update revision with system agent in CC.

Reviewers: epriestley

Reviewed By: epriestley

CC: michalburger1, aran

Differential Revision: https://secure.phabricator.com/D2100