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