Fix write to undefined property $_hunks

Summary: Some time long in the past, this was renamed to unsavedHunks. Fixes T2249.

Test Plan: Ran destroy_revision.php D20, got a successful destruction.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2249

Differential Revision: https://secure.phabricator.com/D4304