Fix for showing an unregistered author name in the tooltip of diffusion revision link

Summary:
When showing contents of a file with the blame mode enabled, tooltips pops out
when the mouse hovers over previous commit linkes on left side. The last part of the
tooltips is the author's name. If an author is unregistered, the name becomes
<span>name</span>.

{F147724}

This doesn't happen if the author is registered.

Test Plan:
Check tooltips after making the change.
{F147725}

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8869