Handle GRPC::NotFound in Gitaly tree_entry call

For https://gitlab.com/gitlab-org/gitaly/-/merge_requests/3078, which is
planned for GitLab 14.0.

lib/gitlab/gitaly_client/commit_service.rb

@@ -107,6 +107,8 @@ module Gitlab
         entry.data = data.join
 
         entry unless entry.oid.blank?
+      rescue GRPC::NotFound
+        nil
       end
 
       def tree_entries(repository, revision, path, recursive)