Fix bad tables in diffusion blame

Summary: My fake data was 100%, and not all tables have full revision history. This leads to a broken table. Instead check if we have any revisions at all, then always show the column, with or without a link inside.

Test Plan: going on a limb this is the correct fix and test on secure... again ...

Reviewers: epriestley

Reviewed By: epriestley

Spies: Korvin

Differential Revision: https://secure.phabricator.com/D18474