Support blame on blame for svn

Summary:
add the column for the blame on blame for svn. We will support
git once we have the 'parent' info of the commits saved in the database
for git.

Test Plan:
in svn it should work. In git is doesn't break things.

Reviewed By: epriestley
Reviewers: epriestley
CC: epriestley
Differential Revision: 95