Get all changed file paths for one or more refs from Gitaly
If we had an RPC that returns paths for all changed files in one or more refs this would allow us to speed up the pre-receive hooks (https://gitlab.com/gitlab-org/gitlab-ce/issues/43281) and the hooks we have in GitLab (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26726#note_157401062).
Right now we are getting the entire delta for a ref, while we only need the path for these checks