Move commit hash querying to DiffusionLowLevelCommitQuery

Summary: Ref T4195. I need this for the Herald pre-commit rules, and it generally simplifies things.

Test Plan: Used `reparse.php` plus `var_dump()` to inspect refs in Git, Mercurial and SVN repos. They all looked correct and reparsed correctly.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7804

src/applications/diffusion/data/DiffusionCommitHash.php

@@ -0,0 +1,26 @@
+<?php
+
+final class DiffusionCommitHash extends Phobject {
+
+  private $hashType;
+  private $hashValue;
+
+  public function setHashValue($hash_value) {
+    $this->hashValue = $hash_value;
+    return $this;
+  }
+
+  public function getHashValue() {
+    return $this->hashValue;
+  }
+
+  public function setHashType($hash_type) {
+    $this->hashType = $hash_type;
+    return $this;
+  }
+
+  public function getHashType() {
+    return $this->hashType;
+  }
+
+}