Add a "Comment content" field to Herald

Summary: Ref T13583. To improve support for making it harder to improperly mix data retention policies, allow Herald to act on comment content.

Test Plan:
  - Wrote comment content Herald rules in Maniphest and Differential.
  - Submitted non-matching comments (no action) and matching comments (Herald action).
  - In Differential, triggered rules by submitting non-matching main content and a matching inline comment.

Maniphest Tasks: T13583

Differential Revision: https://secure.phabricator.com/D21479

src/__phutil_library_map__.php

@@ -1557,6 +1557,7 @@ phutil_register_library_map(array(
     'HeraldBuildableState' => 'applications/herald/state/HeraldBuildableState.php',
     'HeraldCallWebhookAction' => 'applications/herald/action/HeraldCallWebhookAction.php',
     'HeraldCommentAction' => 'applications/herald/action/HeraldCommentAction.php',
+    'HeraldCommentContentField' => 'applications/herald/field/HeraldCommentContentField.php',
     'HeraldCommitAdapter' => 'applications/diffusion/herald/HeraldCommitAdapter.php',
     'HeraldCondition' => 'applications/herald/storage/HeraldCondition.php',
     'HeraldConditionTranscript' => 'applications/herald/storage/transcript/HeraldConditionTranscript.php',
@@ -7770,6 +7771,7 @@ phutil_register_library_map(array(
     'HeraldBuildableState' => 'HeraldState',
     'HeraldCallWebhookAction' => 'HeraldAction',
     'HeraldCommentAction' => 'HeraldAction',
+    'HeraldCommentContentField' => 'HeraldField',
     'HeraldCommitAdapter' => array(
       'HeraldAdapter',
       'HarbormasterBuildableAdapterInterface',

src/applications/herald/field/HeraldCommentContentField.php

@@ -0,0 +1,43 @@
+<?php
+
+final class HeraldCommentContentField extends HeraldField {
+
+  const FIELDCONST = 'comment.content';
+
+  public function getHeraldFieldName() {
+    return pht('Comment content');
+  }
+
+  public function getFieldGroupKey() {
+    return HeraldTransactionsFieldGroup::FIELDGROUPKEY;
+  }
+
+  public function getHeraldFieldValue($object) {
+    $adapter = $this->getAdapter();
+
+    $xactions = $adapter->getAppliedTransactions();
+
+    $result = array();
+    foreach ($xactions as $xaction) {
+      if (!$xaction->hasComment()) {
+        continue;
+      }
+
+      $comment = $xaction->getComment();
+      $content = $comment->getContent();
+
+      $result[] = $content;
+    }
+
+    return $result;
+  }
+
+  public function supportsObject($object) {
+    return true;
+  }
+
+  protected function getHeraldFieldStandardType() {
+    return self::STANDARD_TEXT_LIST;
+  }
+
+}