Pass the Gitlab::Git::Branch object to Repository#merged_to_root_ref?

Signed-off-by: Rémy Coutable <remy@rymai.me>

lib/api/entities.rb

@@ -242,10 +242,7 @@ module API
       end
 
       expose :merged do |repo_branch, options|
-        # n+1: https://gitlab.com/gitlab-org/gitlab-ce/issues/37442
-        Gitlab::GitalyClient.allow_n_plus_1_calls do
-          options[:project].repository.merged_to_root_ref?(repo_branch.name, options[:merged_branch_names])
-        end
+        options[:project].repository.merged_to_root_ref?(repo_branch, options[:merged_branch_names])
       end
 
       expose :protected do |repo_branch, options|