Add a test for the scenario described in:

https://gitlab.com/gitlab-org/gitlab-ee/issues/260#note_12001370

Source branch:
https://gitlab.com/gitlab-org/gitlab-test/tree/merged-target

Target branch:
https://gitlab.com/gitlab-org/gitlab-test/tree/improve/awesome

features/project/ff_merge_requests.feature

@@ -6,7 +6,7 @@ Feature: Project Ff Merge Requests
     And merge request "Bug NS-05" is mergeable
 
   @javascript
-  Scenario: I do ff-only merge
+  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"
@@ -14,6 +14,15 @@ Feature: Project Ff Merge Requests
     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
+
   @javascript
   Scenario: I do rebase before ff-only merge
     Given ff merge enabled

features/steps/project/ff_merge_requests.rb

@@ -58,6 +58,13 @@ class Spinach::Features::ProjectFfMergeRequests < Spinach::FeatureSteps
     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_code
+    merge_request.save!
+  end
+
   step 'rebase before merge enabled' do
     project = merge_request.target_project
     project.merge_requests_rebase_enabled = true