Increase double click target in blame

Summary:
I need to select author or revision quite often.
It's currently almost impossible because double click opens the link and drag-and-dropping tries to move the link.

Test Plan: Double clicked in blame.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3816