Migrate CommitStats to Go

This RPC is implemented in gitaly-ruby and we have indications that it causes performance problems there. https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/5949#note_132687723

We should migrate this RPC to Go.

I think it can be implemented with git diff --numstat.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information