Fix a Diffusion issue where commits that do not show changesets would incorrectly try to render changesets

Summary:
See https://discourse.phabricator-community.org/t/loading-certain-svn-commits-cause-unhandled-exception/3795/.

Commits with no changesets (for example, deleted commits) don't generate a "$changesets".

Test Plan: Viewed a commit with no changesets. Before change: exception. After change: saw unusual commit state.

Differential Revision: https://secure.phabricator.com/D21175