Gitlab::Git::Repository#log is really slow

Dashboard: http://yorick-influxdb.gitlap.com/dashboard/db/gitlab-git-repository-log

Looking at the method this appears to be running git log on the repository: https://gitlab.com/gitlab-org/gitlab_git/blob/2f0d3c1a1337e3f6d67ab0daaf89925d7164d6f0/lib/gitlab_git/repository.rb#L236-275

While I'm not sure if it's actually faster I'm pretty sure we can replicate this in Rugged instead of having to shell out.