Don't link files with no rendering changeset in DiffusionCommitChangeTableView

Summary:
When directories are added (e.g., on the hg initial commit, "/" is added) we don't render any diff for them but try to link to it in the table of contents.

Possibly we should render a diff saying "this directory was added", but stop it from complaining for now.

Test Plan: Looked at several hg and git commits which add directories to verify this gives us generally sensible behvior.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D3663