Summary: Fixes T12090. In obscure situations lost to the mists of time, the `changes` column could be `null`. Force a string cast so the migration finishes, even though these changesets are likely meaningless.
Test Plan:
I did a force-reapply as a sanity check:
```
$ ./bin/storage upgrade -f --apply phabricator:20161213.diff.01.hunks.php
```
That went cleanly; it would only have caught dramatic errors, but I didn't completely butcher things.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T12090
Differential Revision: https://secure.phabricator.com/D17168
Summary:
Ref T8475. This forces installs to migrate hunks to the modern format.
We stopped writing to the legacy format a very long time ago (2+ years?) without issues.
This doesn't destroy any data. T8623 has guidance and I'll publish more changelog guidance.
Test Plan: Faked some legacy data and migrated it.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T8475
Differential Revision: https://secure.phabricator.com/D17039
