Migrate Differential revision edges to use modern EdgeType subclasses

Summary: Modernize Differential edges to subclass `PhabricatorEdgeType`. Largely based on D11045.

Test Plan: From previous experience, these changes are fairly trivial and safe. I poked around a little to make sure things looked reasonably okay.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, Krenair, epriestley

Differential Revision: https://secure.phabricator.com/D11074

resources/sql/patches/migrate-differential-dependencies.php

@@ -18,7 +18,7 @@ foreach (new LiskMigrationIterator($table) as $rev) {
   foreach ($deps as $dep) {
     $editor->addEdge(
       $rev->getPHID(),
-      PhabricatorEdgeConfig::TYPE_DREV_DEPENDS_ON_DREV,
+      DifferentialRevisionDependsOnRevisionEdgeType::EDGECONST,
       $dep);
   }
   $editor->save();