Fix and expand Gitaly FindCommit caching

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26248 added support for deduplicating FindCommit requests using Gitaly ref name caching. However, not all endpoints were covered, and in one case the Gitaly wrapper wasn't actually surrounding the serialization step. We can safely cache ref names between FindCommit calls for #index and #show endpoints for merge requests and pipelines. This can significantly reduce the number of FindCommit requests.