R1:87c6c270b435

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…