Use Redis cache for branch existence checks

app/models/repository.rb

@@ -217,11 +217,7 @@ class Repository
   def branch_exists?(branch_name)
     return false unless raw_repository
-    @branch_exists_memo ||= Hash.new do |hash, key|
-      hash[key] = raw_repository.branch_exists?(key)
-    end
-
-    @branch_exists_memo[branch_name]
+    branch_names.include?(branch_name)
   end
   def ref_exists?(ref)

spec/models/repository_spec.rb

@@ -1166,6 +1166,31 @@ describe Repository do
     end
   end
+  describe '#branch_exists?' do
+    it 'uses branch_names' do
+      allow(repository).to receive(:branch_names).and_return(['foobar'])
+
+      expect(repository.branch_exists?('foobar')).to eq(true)
+      expect(repository.branch_exists?('master')).to eq(false)
+    end
+  end
+
+  describe '#branch_names', :use_clean_rails_memory_store_caching do
+    let(:fake_branch_names) { ['foobar'] }
+
+    it 'gets cached across Repository instances' do
+      allow(repository.raw_repository).to receive(:branch_names).once.and_return(fake_branch_names)
+
+      expect(repository.branch_names).to eq(fake_branch_names)
+
+      fresh_repository = Project.find(project.id).repository
+      expect(fresh_repository.object_id).not_to eq(repository.object_id)
+
+      expect(fresh_repository.raw_repository).not_to receive(:branch_names)
+      expect(fresh_repository.branch_names).to eq(fake_branch_names)
+    end
+  end
+
   describe '#update_autocrlf_option' do
     describe 'when autocrlf is not already set to :input' do
       before do