Fix email for closing a blocking task

Summary: Fixes T5751. Currently, we incorrectly overwrite $xactions.

Test Plan: Closed a blocking task, got an email about the correct transaction set ("closed task; added comment") instead of an overwritten transaction set ("closed blocking task").

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5751

Differential Revision: https://secure.phabricator.com/D10088