Fix interaction between undo and inline comment placement

Summary:
Fixes T7658. Currently, we remove the "undo" before placing the comment, but that causes us to lose track of which row we should be examining.

Instead, place the comment first, then remove the "undo".

Test Plan: This stuff is hard to test comprehensively, but the original report reproduced easily and is now fixed. I wasn't able to break anything by adding/editing/deleting comments.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T7658

Differential Revision: https://secure.phabricator.com/D12157