Fix an issue when undoing mutiple inline comment deletions

Summary:
Ref T13559. If you create comments A and B, then delete comments A and B, then undo the deletion of A, the UI undoes the deletion of B instead.

This is becasue the undo rows are shipped down with a static scalar metadata reference. When copied multiple times to create multiple undo rows, they reference the same data object.

Preventing this in the general case is a problem with greater scope. For now, just avoid rendering these rows with any metadata so they don't alias a single data object.

Test Plan:

• Created comments A, B.
• Deleted comments A, B.
• Clicked "Undo" on A.
  • Before: Deletion of "B" undone.
  • After: Deletion of "A" undone.

Maniphest Tasks: T13559

Differential Revision: https://secure.phabricator.com/D21650