Give audits a clickable commit title, even if they are importing or messageless

Summary:
Fixes T3854. Subversion allows commits with no message, and in other cases we might not have imported the message yet. In these cases, we may not render any text inside the link.

When we hit these cases, render appropriate replacement text.

Test Plan:

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T3854

Differential Revision: https://secure.phabricator.com/D9169