Set "importStatus" as nonmutable on save()

Summary: Fixes T6840. Depends on D11822, which is a little iffy.

Test Plan:
Verified all references to `importStatus` are either:

- SQL patches creating the column;
- reads;
- writes immediately before an insert; or
- explicit updates of the column.

That is, I identified no cases of `setImportStatus(X)->save()` on a Commit which may already exist. This //would// break that.

In general, almost all writes go through `$commit->writeImportStatusFlag()`, which is an explicit update.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6840

Differential Revision: https://secure.phabricator.com/D11823