Add a retroactive migration to expand the `contentHash` field

Summary:
See D18037. The migration there may cause us to write new file records as a side effect.

Ideally, we would rewrite that migration to not ever have this kind of side effect. However, that would make it much more complicated, and it's already very complicated.

Instead, retroactively expand the size of this field before `storage adjust` does it, so it has the right size by the time we hit the migration in D18037.

Test Plan:
@chad, can you `arc patch` this and see if it works?

It's possible that it will get us about five lines deeper and then we'll just hit another similar exception, and that this isn't really a viable way forward.

Reviewers: chad, amckinley

Reviewed By: amckinley

Subscribers: amckinley, chad

Differential Revision: https://secure.phabricator.com/D18107