Fix json response in branches controller

app/controllers/projects/branches_controller.rb

@@ -20,7 +20,7 @@ class Projects::BranchesController < Projects::ApplicationController
     respond_to do |format|
       format.html
       format.json do
-        render json: @repository.branch_names
+        render json: @branches.map(&:name)
       end
     end
   end

changelogs/unreleased/28898-fix-search-branches-in-cherry-picking.yml

+---
+title: Fix json response in branches controller
+merge_request: 9710
+author: George Andrinopoulos

spec/controllers/projects/branches_controller_spec.rb

@@ -244,4 +244,27 @@ describe Projects::BranchesController do
       end
     end
   end
+
+  describe "GET index" do
+    render_views
+
+    before do
+      sign_in(user)
+    end
+
+    context 'when rendering a JSON format' do
+      it 'filters branches by name' do
+        get :index,
+            namespace_id: project.namespace,
+            project_id: project,
+            format: :json,
+            search: 'master'
+
+        parsed_response = JSON.parse(response.body)
+
+        expect(parsed_response.length).to eq 1
+        expect(parsed_response.first).to eq 'master'
+      end
+    end
+  end
 end