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