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