R1:c9889e3d55b1

Fix an issue in Phriction where moving a document just copied it instead

Summary: Ref T12732. See D17918. With modular transactions, `getCustomTransactionNewValue()` isn't actually called.

Test Plan: Moved document `/x/` to `/y/`, saw document gone at `/x/` instead of copied.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12732

Differential Revision: https://secure.phabricator.com/D17963