Parse Tags in commits message for revisions

Summary: This will stop breaking if you have subscribers and tags when updating a revision (Error parsing field "Subscribers": The objects you have listed include objects which do not exist (Tags:)), which I broke in D15749.

Test Plan: run through arc-diff --update that failed earlier.

Reviewers: chad, #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15762