Simplify timeline rendering on Differential revisions

Summary: Ref T7447. Remove the loadInlineComments() by-ref side-effect mess and make this load more direct.

Test Plan:

• Viewed a revision.
• Clicked "Show Older".

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7447

Differential Revision: https://secure.phabricator.com/D12496