Use a query in Project#protected_branch?

This changes Project#protected_branch? to use a query to check if a
branch is protected, instead of loading all ProtectedBranch records into
memory just to check if the list of names includes a given branch name.

app/models/project.rb

@@ -764,7 +764,7 @@ class Project < ActiveRecord::Base
 
   # Check if current branch name is marked as protected in the system
   def protected_branch?(branch_name)
-    protected_branches_names.include?(branch_name)
+    protected_branches.where(name: branch_name).any?
   end
 
   def developers_can_push_to_protected_branch?(branch_name)

spec/models/project_spec.rb

@@ -798,4 +798,18 @@ describe Project, models: true do
       end
     end
   end
+
+  describe '#protected_branch?' do
+    let(:project) { create(:empty_project) }
+
+    it 'returns true when a branch is a protected branch' do
+      project.protected_branches.create!(name: 'foo')
+
+      expect(project.protected_branch?('foo')).to eq(true)
+    end
+
+    it 'returns false when a branch is not a protected branch' do
+      expect(project.protected_branch?('foo')).to eq(false)
+    end
+  end
 end