Fix a "Wrong number of arguments" error in TreeHelper#flatten_tree

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

app/helpers/tree_helper.rb

@@ -104,7 +104,7 @@ module TreeHelper
 
     subtree = Gitlab::Git::Tree.where(@repository, @commit.id, tree.path)
     if subtree.count == 1 && subtree.first.dir?
-      return tree_join(tree.name, flatten_tree(subtree.first))
+      return tree_join(tree.name, flatten_tree(root_path, subtree.first))
     else
       return tree.name
     end

spec/features/projects/user_browses_a_tree_with_a_folder_containing_only_a_folder.rb

@@ -0,0 +1,20 @@
+require 'spec_helper'
+
+# This is a regression test for https://gitlab.com/gitlab-org/gitlab-ce/issues/37569
+describe 'User browses a tree with a folder containing only a folder' do
+  let(:project) { create(:project, :empty_repo) }
+  let(:user) { project.creator }
+
+  before do
+    # We need to disable the tree.flat_path provided by Gitaly to reproduce the issue
+    allow(Gitlab::GitalyClient).to receive(:feature_enabled?).and_return(false)
+
+    project.repository.create_dir(user, 'foo/bar', branch_name: 'master', message: 'Add the foo/bar folder')
+    sign_in(user)
+    visit(project_tree_path(project, project.repository.root_ref))
+  end
+
+  it 'shows the nested folder on a single row' do
+    expect(page).to have_content('foo/bar')
+  end
+end