Fix "context not available" in contextless diffs

Summary: D4351 swapped this from a map to a list, and then stopped it from getting to the ChangesetParser so it didn't make it to the Renderer.

Test Plan: Ran "arc diff", copy/pasted it into the web UI. Saw a diff with "context not available" blocks in between missing contexts.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D4384