Use LiskDAOSet's loadRelativeEdges with ReleephRequest

Summary:
`ReleephRequest`s contain the PHID for a `PhabricatorRepositoryCommit`, and commits have an edge to a `DifferentialRevision`.

Commits are loaded with the `loadOneRelative()` method that loads the commits for every `ReleephRequest` in a `LiskDAOSet`, but the edges are loaded indivdually.  A page with N RQs on it makes one DB query for the commits, but N queries for the `TYPE_COMMIT_HAS_DREV` edges.

This diff uses `loadRelativeEdges` instead to load the edges all in one query.

Test Plan: {F42290}

Reviewers: wez, epriestley

Reviewed By: epriestley

CC: epriestley, vrana, aran

Maniphest Tasks: T2714

Differential Revision: https://secure.phabricator.com/D5820

src/applications/releeph/storage/ReleephRequest.php

@@ -228,10 +228,13 @@ final class ReleephRequest extends ReleephDAO {
   }
 
   public function loadRequestCommitDiffPHID() {
-    $revision_phid = PhabricatorEdgeQuery::loadDestinationPHIDs(
-      $this->getRequestCommitPHID(),
-      PhabricatorEdgeConfig::TYPE_COMMIT_HAS_DREV);
-    return reset($revision_phid);
+    $commit = $this->loadPhabricatorRepositoryCommit();
+    if ($commit) {
+      $edges = $this
+        ->loadPhabricatorRepositoryCommit()
+        ->loadRelativeEdges(PhabricatorEdgeConfig::TYPE_COMMIT_HAS_DREV);
+      return head(array_keys($edges));
+    }
   }