Don't require mentioned objects to have all required fields when editing comments

Summary: Fixes T12439. This pathway was just missing a `setContinueOnMissingFields(...)` to skip enforcement of required fields.

Test Plan:
  - Added a required custom field.
  - Mentioned any task without a field value in a comment.
  - Edited that comment.
  - Saved changes.
  - Before fix: fatal in log.
  - After fix: clean edit.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12439

Differential Revision: https://secure.phabricator.com/D17536

src/applications/transactions/editor/PhabricatorApplicationTransactionCommentEditor.php

@@ -92,6 +92,7 @@ final class PhabricatorApplicationTransactionCommentEditor
               $editor
                 ->setContentSource($this->getContentSource())
                 ->setContinueOnNoEffect(true)
+                ->setContinueOnMissingFields(true)
                 ->applyTransactions($object, $support_xactions);
             }
           }