Add a (dst, type, src) key to Differential's edge table

Summary:
Ref T1279. This came to me in a dream.

The existing `differential_relationship` table has an `(objectPHID, type)` column, which theoretically is useful for queries like "revisions with X as a reviewer". In practice, I'm not sure it gets used much, but I can get it to show up in at least some query plans.

Add a similar index to the `edge` table. This sequences //before// D7227, which actually migrates the data.

Test Plan:
  - Ran `bin/storage upgrade`.
  - EXPLAIN'd a bunch of queries against different versions of the schema, this seemed helpful overall.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1279

Differential Revision: https://secure.phabricator.com/D7232

Conflicts:
	src/infrastructure/storage/patch/PhabricatorBuiltinPatchList.php

resources/sql/patches/20131004.dxedgekey.sql

@@ -0,0 +1,2 @@
+ALTER TABLE {$NAMESPACE}_differential.edge
+  ADD UNIQUE KEY `key_dst` (dst, type, src);

src/infrastructure/storage/patch/PhabricatorBuiltinPatchList.php

@@ -1652,6 +1652,14 @@ final class PhabricatorBuiltinPatchList extends PhabricatorSQLPatchList {
         'type' => 'sql',
         'name' => $this->getPatchPath('20130929.filepolicy.sql'),
       ),
+      '20131004.dxedgekey.sql' => array(
+        'type' => 'sql',
+        'name' => $this->getPatchPath('20131004.dxedgekey.sql'),
+      ),
+      '20131004.dxreviewers.php' => array(
+        'type' => 'php',
+        'name' => $this->getPatchPath('20131004.dxreviewers.php'),
+      ),
     );
   }
 }