Reorder "Merge" transaction to make "Close as Duplicate" produce a "[Merged]" email subject

Summary:
Fixes T11782. When you "Close as Duplicate", generate a "[Merged]" email by making the merge the first transaction.

(There are other, more-deterministic ways to do this with action strength, but this is much simpler and I believe it suffices.)

Test Plan: Used "Close as Duplicate", got a "[Merged]" email out of it.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T11782

Differential Revision: https://secure.phabricator.com/D19972