Enrich the "change project tags" transaction in "transaction.search"

Summary:
Depends on D20208. Ref T13255. See that task for some long-winded discussion and rationale. Short version:

  - This is a list of operations instead of a list of old/new PHIDs because of scalability issues for large lists (T13056).
  - This is a fairly verbose list (instead of, for example, the more concise internal map we sometimes use with "+" and "-" as keys) to try to make the structure obvious and extensible in the future.
  - The "add" and "remove" echo the `*.edit` operations.

Test Plan: Called `transaction.search` on an object with project tag changes, saw them in the results.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13255

Differential Revision: https://secure.phabricator.com/D20209

src/applications/transactions/conduit/TransactionSearchConduitAPIMethod.php

@@ -202,6 +202,14 @@ final class TransactionSearchConduitAPIMethod
           case PhabricatorTransactions::TYPE_CREATE:
             $type = 'create';
             break;
+          case PhabricatorTransactions::TYPE_EDGE:
+            switch ($xaction->getMetadataValue('edge:type')) {
+              case PhabricatorProjectObjectHasProjectEdgeType::EDGECONST:
+                $type = 'projects';
+                $fields = $this->newEdgeTransactionFields($xaction);
+                break;
+            }
+            break;
         }
       }
 
@@ -264,5 +272,29 @@ final class TransactionSearchConduitAPIMethod
     return $query;
   }
 
+  private function newEdgeTransactionFields(
+    PhabricatorApplicationTransaction $xaction) {
+
+    $record = PhabricatorEdgeChangeRecord::newFromTransaction($xaction);
+
+    $operations = array();
+    foreach ($record->getAddedPHIDs() as $phid) {
+      $operations[] = array(
+        'operation' => 'add',
+        'phid' => $phid,
+      );
+    }
+
+    foreach ($record->getRemovedPHIDs() as $phid) {
+      $operations[] = array(
+        'operation' => 'remove',
+        'phid' => $phid,
+      );
+    }
+
+    return array(
+      'operations' => $operations,
+    );
+  }
 
 }