Un-breaking the update_phabricator.sh script for those that have data in their chatlog_event table

Summary:
The last commit broke update_phabricator.sh for me when it tried to migrate channel names into its own table.

It's a fairly straight forward patch and I'm almost certain I fixed it correctly. :)

Test Plan: Made small changes, ran update_phabricator.sh, repeat until the errors went away.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5116