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

src/applications/differential/storage/DifferentialChangeset.php

@@ -88,7 +88,7 @@ final class DifferentialChangeset extends DifferentialDAO {
       foreach ($this->loadHunks() as $hunk) {
         $hunk->delete();
       }
-      $this->_hunks = array();
+      $this->unsavedHunks = array();
 
       queryfx(
         $this->establishConnection('w'),