R1:63bfad0ff476

Refine unusual inline comment client interactions

Summary: Ref T13513. Refine some inline behaviors, see test plan.

Test Plan:
- Edit a comment ("A"), type text ("AB"), cancel, edit.
- Old behavior: edit and undo states (wrong, and undo does not function).
- New behavior: edit state only.
- Edit a comment ("A"), type text ("AB"), cancel. Undo ("AB"), cancel. Edit.
- Old behavior: "AB" (wrong: you never submitted this text).
- New behavior: "A".
- Create a comment, type text, cancel.