Fix "left"/"right" changeset ID selection for synthetic deletions

Summary:
Fixes T8323. See that task for a description.

We were using nonempty(), but that rule doesn't cover synthetic deletions (file present in an earlier diff, but no longer present in the later diff).

Test Plan: Followed the steps in T8323, got a clean comment.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8323

Differential Revision: https://secure.phabricator.com/D17929