Fix accidental forward dependency in old commit summary migration

Summary: See IRC. This migration inadvertently depends on the columns in the commit table, because it calls save(), and thus broke for installs with data after we added the importStatus column. Since that was ~9 months after this patch, probably not many installs are affected.

Test Plan: Ran patch locally with --apply on data. Had user verify fix.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D8152