make commit message worker savvier around revision ids
Summary: Fixes T2836
Test Plan: make a diff, get it approved, arc land, verify things okay. ask users on T2836 to try.
Reviewers: epriestley
Reviewed By: epriestley
CC: Korvin, aran
Maniphest Tasks: T2836
Differential Revision: https://secure.phabricator.com/D6770