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