Forking repositories is handled by Gitaly by default

Initial Rails implementation was introduced in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16018, and
acceptance testing was done in
https://gitlab.com/gitlab-org/gitaly/issues/881

Part of https://gitlab.com/gitlab-org/gitaly/issues/817

changelogs/unreleased/zj-fork-opt-out.yml

+---
+title: Gitaly handles repository forks by default
+merge_request:
+author:
+type: other

lib/gitlab/git/gitlab_projects.rb

@@ -63,7 +63,8 @@ module Gitlab
       end
       def fork_repository(new_shard_name, new_repository_relative_path)
-        Gitlab::GitalyClient.migrate(:fork_repository) do |is_enabled|
+        Gitlab::GitalyClient.migrate(:fork_repository,
+                                     status: Gitlab::GitalyClient::MigrationStatus::OPT_OUT) do |is_enabled|
           if is_enabled
             gitaly_fork_repository(new_shard_name, new_repository_relative_path)
           else