Read rename information from DB instead of repository

Summary:
We already have this information for all VCSes, there's no point in parsing it again.
We currently support it only for Git and I remember there were some bugs in it.
It should also be faster.

Test Plan: Blamed previous revision of a file which was moved in SVN.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3564