Add and populate a `pathIndex` column for OwnersPath

Summary: Ref T11015. This supports making path names arbitrarily long and putting a proper unique key on the table.

Test Plan:
- Migrated, checked database, saw nice digested indexes.
- Edited a package, saw new rows update with digested indexes.

Maniphest Tasks: T11015

Differential Revision: https://secure.phabricator.com/D19181

resources/sql/autopatches/20180306.opath.01.digest.sql

···
+ALTER TABLE {$NAMESPACE}_owners.owners_path
+  ADD pathIndex BINARY(12) NOT NULL;

resources/sql/autopatches/20180306.opath.02.digestpopulate.php

···
+<?php
+
+$table = new PhabricatorOwnersPath();
+$conn = $table->establishConnection('w');
+
+foreach (new LiskMigrationIterator($table) as $path) {
+  $index = PhabricatorHash::digestForIndex($path->getPath());
+
+  if ($index === $path->getPathIndex()) {
+    continue;
+  }
+
+  queryfx(
+    $conn,
+    'UPDATE %T SET pathIndex = %s WHERE id = %d',
+    $table->getTableName(),
+    $index,
+    $path->getID());
+}

src/applications/owners/storage/PhabricatorOwnersPath.php

···
 
   protected $packageID;
   protected $repositoryPHID;
+  protected $pathIndex;
   protected $path;
   protected $excluded;
 
···
       self::CONFIG_TIMESTAMPS => false,
       self::CONFIG_COLUMN_SCHEMA => array(
         'path' => 'text255',
+        'pathIndex' => 'bytes12',
         'excluded' => 'bool',
       ),
       self::CONFIG_KEY_SCHEMA => array(
···
       ),
     ) + parent::getConfiguration();
   }
-
 
   public static function newFromRef(array $ref) {
     $path = new PhabricatorOwnersPath();
     $path->repositoryPHID = $ref['repositoryPHID'];
-    $path->path = $ref['path'];
+
+    $raw_path = $ref['path'];
+
+    $path->pathIndex = PhabricatorHash::digestForIndex($raw_path);
+    $path->path = $raw_path;
+
     $path->excluded = $ref['excluded'];
+
     return $path;
   }