Repository Exists check is OPT_OUT for Gitaly

Moving more git operations to be executed by Gitaly, now the check if a
repository exists is an opt out endpoint.

Can be disabled, for the time being, by performing in the rails console:
> Feature.get('gitaly_repository_exists').disable
=> true

Part of gitlab-org/gitaly#314

lib/gitlab/git/repository.rb

@@ -105,7 +105,7 @@ module Gitlab
       end
 
       def exists?
-        Gitlab::GitalyClient.migrate(:repository_exists) do |enabled|
+        Gitlab::GitalyClient.migrate(:repository_exists, status: Gitlab::GitalyClient::MigrationStatus::OPT_OUT) do |enabled|
           if enabled
             gitaly_repository_client.exists?
           else