Fix an issue where inlines could be duplicated in the client list

Summary:
Ref T13559. D21261 added caching here, but the logic in rebuilding inlines wasn't quite correct, and could lead to us double-appending.

Instead, when rebuilding, unconditionally discard the old list.

Test Plan:

• Added inline comments to a file in Differential.
• Marked some done.
• Scrolled so the inline comment header was visible, saw "X / Y Comments" button in header.
• Clicked "Show 20 more lines" on the changeset with inlines (or toggle "View Unified" / "View Side-by-Side", or other interactions likely work too).
  • Before: saw "X / Y" change improperly (because inlines in that file were double-counted).
  • After: saw stable count.
• Grepped for "differential-inline-comment-refresh", got no hits, concluded this event has no listeners.

Maniphest Tasks: T13559

Differential Revision: https://secure.phabricator.com/D21642