Add GetObjectDirectorySize RPC
This RPC will be used to determine if a push will exceed the size limit of a repository.
gitaly MR: gitaly!1263 (merged)
gitlab EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13460
EE issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/11126
Edited by Patrick Bajao