Instantiate the stub using GitalyClient.stub instead of the old way

lib/gitlab/gitaly_client/commit.rb

@@ -27,7 +27,7 @@ module Gitlab
         def diff_from_parent(commit, options = {})
           repository = commit.project.repository
           gitaly_repo = repository.gitaly_repository
-          stub = Gitaly::Diff::Stub.new(nil, nil, channel_override: repository.gitaly_channel)
+          stub = GitalyClient.stub(:diff, repository.storage)
           parent = commit.parents[0]
           parent_id = parent ? parent.id : EMPTY_TREE_ID
           request = Gitaly::CommitDiffRequest.new(