R1:e8d3071452f0

Implement a git blame cache

Summary: Ref T2450. Ref T2453. Add a repository_blamecache table and cache git blame information

Test Plan: View files in Diffusion with enabled blame

Reviewers: fabe, chad, #blessed_reviewers

Reviewed By: chad, #blessed_reviewers

Subscribers: joshuaspence, epriestley

Maniphest Tasks: T2453, T2450

Differential Revision: https://secure.phabricator.com/D10600