Treat MERGE_INTO as a status transaction for mail tags

Summary: Fixes T6838. We use a special transaction type for merging, but don't handle it when figuring out mail tags.

Test Plan: Verified merge mail picks up the maniphest-status tag.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T6838

Differential Revision: https://secure.phabricator.com/D11101