Reorder "Merge" transaction to make "Close as Duplicate" produce a "[Merged]" email subject
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.
Reviewed By: amckinley
Maniphest Tasks: T11782
Differential Revision: https://secure.phabricator.com/D19972