Projects::CompareController#show is calling Gitaly n+1 times per request

Projects::CompareController#show is calling Gitaly n+1 times per request.

This code needs to be refactored to fetch all required information in a single call.

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13996/diffs#168dd0871371b181661cbd02e08506b2797fdc71_19_19