Extracted ProtectableDropdown to clean up Project#open_branches

Makes it clear this is only used in dropdowns, instead of cluttering up Project class. Since we only care about branch names, it is also possible to refactor out a lot of the set/reject logic.

A benchmark on Array/Set subtraction favoured using Arrays. This was with 5000 ‘branches’ and 2000 ‘protections’ to ensure a similar comparison to the commit which introduced using Set for intersection.

Comparison:
   array subtraction:      485.8 i/s
     set subtraction:      128.7 i/s - 3.78x slower

2 files changed, 24 insertions, 19 deletions

diff --git a/spec/models/project_spec.rb b/spec/models/project_spec.rb
index f68631ebe06..cc06949974e 100644
--- a/spec/models/project_spec.rb
+++ b/spec/models/project_spec.rb
@@ -702,25 +702,6 @@ describe Project, models: true do
     end
   end
 
-  describe '#open_branches' do
-    let(:project) { create(:project, :repository) }
-
-    before do
-      project.protected_branches.create(name: 'master')
-    end
-
-    it { expect(project.open_branches.map(&:name)).to include('feature') }
-    it { expect(project.open_branches.map(&:name)).not_to include('master') }
-
-    it "includes branches matching a protected branch wildcard" do
-      expect(project.open_branches.map(&:name)).to include('feature')
-
-      create(:protected_branch, name: 'feat*', project: project)
-
-      expect(Project.find(project.id).open_branches.map(&:name)).to include('feature')
-    end
-  end
-
   describe '#star_count' do
     it 'counts stars from multiple users' do
       user1 = create :user
diff --git a/spec/models/protectable_dropdown_spec.rb b/spec/models/protectable_dropdown_spec.rb
new file mode 100644
index 00000000000..7f8ef7195e5
--- /dev/null
+++ b/spec/models/protectable_dropdown_spec.rb
@@ -0,0 +1,24 @@
+require 'spec_helper'
+
+describe ProtectableDropdown, models: true do
+  let(:project) { create(:project, :repository) }
+  let(:subject) { described_class.new(project, :branches) }
+
+  describe '#protectable_ref_names' do
+    before do
+      project.protected_branches.create(name: 'master')
+    end
+
+    it { expect(subject.protectable_ref_names).to include('feature') }
+    it { expect(subject.protectable_ref_names).not_to include('master') }
+
+    it "includes branches matching a protected branch wildcard" do
+      expect(subject.protectable_ref_names).to include('feature')
+
+      create(:protected_branch, name: 'feat*', project: project)
+
+      subject = described_class.new(project.reload, :branches)
+      expect(subject.protectable_ref_names).to include('feature')
+    end
+  end
+end