DeleteMergedBranchesService should not delete protected branches

When deleting all the branches that are merged, the protected branches
should not be deleted.

app/services/delete_merged_branches_service.rb

@@ -10,6 +10,8 @@ def execute
     branches = branches.select { |branch| project.repository.merged_to_root_ref?(branch) }
     # Prevent deletion of branches relevant to open merge requests
     branches -= merge_request_branch_names
+    # Prevent deletion of protected branches
+    branches -= project.protected_branches.pluck(:name)
 
     branches.each do |branch|
       DeleteBranchService.new(project, current_user).execute(branch)

changelogs/unreleased/tc-no-delete-protected-merged-branches.yml

+---
+title: Do not delete protected branches when deleting all merged branches
+merge_request: 12624
+author:

spec/services/delete_merged_branches_service_spec.rb

@@ -24,6 +24,14 @@
       expect(project.repository.branch_names).to include('master')
     end
 
+    it 'keeps protected branches' do
+      create(:protected_branch, project: project, name: 'improve/awesome')
+
+      service.execute
+
+      expect(project.repository.branch_names).to include('improve/awesome')
+    end
+
     context 'user without rights' do
       let(:user) { create(:user) }