Reduce code duplication for inline "Undo"

Summary:
Ref T2009. This is another almost-identical copy of the row scaffolding, which has the same 1up/2up bugs as the 8 other copies of this code.

Turn the "undo" element into an InlineCommentView so we can scaffold it.

Then, scaffold it with the same code as everything else.

Test Plan: Hit "Undo", swapped from 1up to 2up, hit "undo" again, swapped back, tried left/right, everything rendered with proper scaffolding.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D12019

src/infrastructure/diff/view/PHUIDiffInlineCommentUndoView.php

@@ -0,0 +1,40 @@
+<?php
+
+/**
+ * Render the "Undo" action to recover discarded inline comments.
+ *
+ * This extends @{class:PHUIDiffInlineCommentView} so it can use the same
+ * scaffolding code as other kinds of inline comments.
+ */
+final class PHUIDiffInlineCommentUndoView
+  extends PHUIDiffInlineCommentView {
+
+  private $isOnRight;
+
+  public function setIsOnRight($is_on_right) {
+    $this->isOnRight = $is_on_right;
+    return $this;
+  }
+
+  public function getIsOnRight() {
+    return $this->isOnRight;
+  }
+
+  public function render() {
+    $link = javelin_tag(
+      'a',
+      array(
+        'href'  => '#',
+        'sigil' => 'differential-inline-comment-undo',
+      ),
+      pht('Undo'));
+
+    return phutil_tag(
+      'div',
+      array(
+        'class' => 'differential-inline-undo',
+      ),
+      array('Changes discarded. ', $link));
+  }
+
+}