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.
  • Old behavior: counter appears in filetree (wrong, comment is undo-able but should not be counted).
  • New behavior: no counter.
• Cancel editing an empty comment with no text.
  • Old behavior: Something buggy -- undo, I think?
  • New behavior: it just vanishes (correct behavior).

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21212