Fixed 'Modified' link in Diffusion History View.

Summary:
'Modified' link always pointed to the latest change. Fixed the link.

Test Plan:
Tested that clicking 'Modified' took me to the correct place. Made sure that
Diffusion commit view was still working correctly.

Reviewed By: epriestley
Reviewers: epriestley
CC: jungejason, aran, epriestley
Differential Revision: 255