Repository#last_commit_for_path N+1
We're seeing up to [100 requests per second](https://performance.gitlab.net/dashboard/db/gitaly-feature-status?from=1513136038338&to=1513179176039&orgId=1&var-job=gitaly-production&var-method=LastCommitForPath) on this RPC which is not good (servers might fall over).
issue