Adjust action strength of self subscribe/unsubscribe

Summary:
Ref T4968. If you add a comment to revision you aren't currently subscribed to, the email currently chooses "[Changed Subscribers]" as the action title. This is less interesting than "[Commented]", provided the affected subscriber is you (adding other people //is// usually interesting).

In this case, reduce the strength of this action below the strength of "comment".

Test Plan: Made several comments in conjunction with implicit and explicit subscriptions. Saw "[Commented]" for stuff affecting me, and "[Changed Subscribers]" for stuff affecting others.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4968

Differential Revision: https://secure.phabricator.com/D9168

src/applications/transactions/storage/PhabricatorApplicationTransaction.php

@@ -780,6 +780,33 @@ abstract class PhabricatorApplicationTransaction
     switch ($this->getTransactionType()) {
       case PhabricatorTransactions::TYPE_COMMENT:
         return 0.5;
+      case PhabricatorTransactions::TYPE_SUBSCRIBERS:
+        $old = $this->getOldValue();
+        $new = $this->getNewValue();
+
+        $add = array_diff($old, $new);
+        $rem = array_diff($new, $old);
+
+        // If this action is the actor subscribing or unsubscribing themselves,
+        // it is less interesting. In particular, if someone makes a comment and
+        // also implicitly subscribes themselves, we should treat the
+        // transaction group as "comment", not "subscribe". In this specific
+        // case (one affected user, and that affected user it the actor),
+        // decrease the action strength.
+
+        if ((count($add) + count($rem)) != 1) {
+          // Not exactly one CC change.
+          break;
+        }
+
+        $affected_phid = head(array_merge($add, $rem));
+        if ($affected_phid != $this->getAuthorPHID()) {
+          // Affected user is someone else.
+          break;
+        }
+
+        // Make this weaker than TYPE_COMMENT.
+        return 0.25;
     }
     return 1.0;
   }