Bugfix for D7596

Summary: Fixed a small bug that caused the catch-all commit to purge previously added symbols in that session.

Test Plan: Re-ran the script, observed corrected behavior.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4117

Differential Revision: https://secure.phabricator.com/D7597