Un-hide the "X added reviewers: ..." transactions in revision creation mail

Summary:
Fixes T12118. See PHI54. This adds a special case for the initial "reviewers" transactions, similar to the existing special case for "projects" transactions.

Although these transactions are redudnant in the web view since you can see the information clearly on the page, they're more reasonably useful in mail.

Test Plan: {F5168838}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12118

Differential Revision: https://secure.phabricator.com/D18542

src/applications/differential/storage/DifferentialTransaction.php

@@ -105,6 +105,18 @@ final class DifferentialTransaction
     return parent::shouldHide();
   }
 
+  public function shouldHideForMail(array $xactions) {
+    switch ($this->getTransactionType()) {
+      case DifferentialRevisionReviewersTransaction::TRANSACTIONTYPE:
+        // Don't hide the initial "X added reviewers: ..." transaction during
+        // object creation from mail. See T12118 and PHI54.
+        return false;
+    }
+
+    return parent::shouldHideForMail($xactions);
+  }
+
+
   public function isInlineCommentTransaction() {
     switch ($this->getTransactionType()) {
       case self::TYPE_INLINE: