Hide squash checkbox when there is a single commit

When there is only one commit, the squash checkbox doesn't do
anything. When a commit is added after the widget is loaded, the MR
can't be merged anyway - you have to refresh the page. So we can only
show the checkbox on page load if there's more than one commit.

app/views/projects/merge_requests/widget/open/_accept.html.haml

@@ -38,9 +38,10 @@
           = label_tag :should_remove_source_branch, class: "merge-param-checkbox" do
             = check_box_tag :should_remove_source_branch
             Remove source branch
+      = hidden_field_tag :squash, '0', id: nil
+      - if @merge_request.commits_count > 1
         .accept-control.checkbox
           = label_tag :squash, class: 'merge-param-checkbox' do
-          = hidden_field_tag :squash, '0', id: nil
             = check_box_tag :squash, '1', @merge_request.squash
             Squash commits
             = link_to icon('question-circle'), help_page_path('user/project/merge_requests/squash_and_merge'), title: 'About this feature', data: {toggle: 'tooltip', placement: 'bottom', container: 'body'}

spec/features/merge_requests/squash_spec.rb

@@ -46,6 +46,18 @@ feature 'Squashing merge requests', js: true, feature: true do
     login_as user
   end
 
+  context 'when the MR has only one commit' do
+    before do
+      merge_request = create(:merge_request, source_project: project, target_project: project, source_branch: 'master', target_branch: 'branch-merged')
+
+      visit namespace_project_merge_request_path(project.namespace, project, merge_request)
+    end
+
+    it 'does not show the squash checkbox' do
+      expect(page).not_to have_field('squash')
+    end
+  end
+
   context 'when squash is enabled on merge request creation' do
     before do
       visit new_namespace_project_merge_request_path(project.namespace, project, merge_request: { target_branch: 'master', source_branch: source_branch })