Allow users to compare branches on a read-only instance

app/views/projects/branches/_branch.html.haml

@@ -53,9 +53,8 @@
         = _('Merge request')
 
     - if branch.name != @repository.root_ref
-      = link_to project_compare_index_path(@project, from: @repository.root_ref, to: branch.name),
+      = link_to project_compare_path(@project, @repository.root_ref, branch.name),
         class: "btn btn-default #{'prepend-left-10' unless merge_project}",
-        method: :post,
         title: s_('Branches|Compare') do
         = s_('Branches|Compare')
 

spec/features/projects/branches_spec.rb

@@ -229,6 +229,38 @@ describe 'Branches' do
     end
   end
 
+  describe 'comparing branches' do
+    before do
+      sign_in(user)
+      project.add_developer(user)
+    end
+
+    shared_examples 'compares branches' do
+      it 'compares branches' do
+        visit project_branches_path(project)
+
+        page.within first('.all-branches li') do
+          click_link 'Compare'
+        end
+
+        expect(page).to have_content 'Commits'
+        expect(page).to have_link 'Create merge request'
+      end
+    end
+
+    context 'on a read-only instance' do
+      before do
+        allow(Gitlab::Database).to receive(:read_only?).and_return(true)
+      end
+
+      it_behaves_like 'compares branches'
+    end
+
+    context 'on a read-write instance' do
+      it_behaves_like 'compares branches'
+    end
+  end
+
   def sorted_branches(repository, count:, sort_by:, state: nil)
     branches = repository.branches_sorted_by(sort_by)
     branches = branches.select { |b| state == 'active' ? b.active? : b.stale? } if state