Port fast-foward spinach specs from EE

features/project/ff_merge_requests.feature

+Feature: Project Ff Merge Requests
+  Background:
+    Given I sign in as a user
+    And I own project "Shop"
+    And project "Shop" have "Bug NS-05" open merge request with diffs inside
+    And merge request "Bug NS-05" is mergeable
+
+  @javascript
+  Scenario: I do ff-only merge for rebased branch
+    Given ff merge enabled
+    And merge request "Bug NS-05" is rebased
+    When I visit merge request page "Bug NS-05"
+    Then I should see ff-only merge button
+    When I accept this merge request
+    Then I should see merged request
+
+  @javascript
+  Scenario: I do ff-only merge for merged branch
+    Given ff merge enabled
+    And merge request "Bug NS-05" merged target
+    When I visit merge request page "Bug NS-05"
+    Then I should see ff-only merge button
+    When I accept this merge request
+    Then I should see merged request

features/steps/project/ff_merge_requests.rb

+class Spinach::Features::ProjectFfMergeRequests < Spinach::FeatureSteps
+  include SharedAuthentication
+  include SharedIssuable
+  include SharedProject
+  include SharedNote
+  include SharedPaths
+  include SharedMarkdown
+  include SharedDiffNote
+  include SharedUser
+  include WaitForRequests
+
+  step 'project "Shop" have "Bug NS-05" open merge request with diffs inside' do
+    create(:merge_request_with_diffs,
+           title: "Bug NS-05",
+           source_project: project,
+           target_project: project,
+           author: project.users.first)
+  end
+
+  step 'I should see ff-only merge button' do
+    expect(page).to have_content "Fast-forward merge without a merge commit"
+    expect(page).to have_button 'Merge'
+  end
+
+  step 'merge request "Bug NS-05" is mergeable' do
+    merge_request.mark_as_mergeable
+  end
+
+  step 'I accept this merge request' do
+    page.within '.mr-state-widget' do
+      click_button "Merge"
+    end
+  end
+
+  step 'I should see merged request' do
+    page.within '.status-box' do
+      expect(page).to have_content "Merged"
+      wait_for_requests
+    end
+  end
+
+  step 'ff merge enabled' do
+    project = merge_request.target_project
+    project.merge_requests_ff_only_enabled = true
+    project.save!
+  end
+
+  step 'merge request "Bug NS-05" is rebased' do
+    merge_request.source_branch = 'flatten-dir'
+    merge_request.target_branch = 'improve/awesome'
+    merge_request.reload_diff
+    merge_request.save!
+  end
+
+  step 'merge request "Bug NS-05" merged target' do
+    merge_request.source_branch = 'merged-target'
+    merge_request.target_branch = 'improve/awesome'
+    merge_request.reload_diff
+    merge_request.save!
+  end
+
+  def merge_request
+    @merge_request ||= MergeRequest.find_by!(title: "Bug NS-05")
+  end
+end