Move Gitaly RefExists check to OPT_OUT

Part of the Gitaly migration as tracked in: https://gitlab.com/gitlab-org/gitaly/issues/401

Acceptance testing was done in: https://gitlab.com/gitlab-org/gitaly/issues/1014

changelogs/unreleased/zj-ref-exists-opt-out.yml

+---
+title: Check if a ref exists is done by Gitaly by default
+merge_request:
+author:
+type: performance

lib/gitlab/git/repository.rb

@@ -297,7 +297,8 @@ module Gitlab
       #
       # Ref names must start with `refs/`.
       def ref_exists?(ref_name)
-        gitaly_migrate(:ref_exists) do |is_enabled|
+        gitaly_migrate(:ref_exists,
+                      status: Gitlab::GitalyClient::MigrationStatus::OPT_OUT) do |is_enabled|
           if is_enabled
             gitaly_ref_exists?(ref_name)
           else