Add MergeRequest#must_be_rebased? and use it in mergeable?

app/controllers/projects/merge_requests_controller.rb

@@ -179,11 +179,6 @@ class Projects::MergeRequestsController < Projects::ApplicationController
       return
     end
 
-    if @project.ff_merge_must_be_possible? && !@merge_request.ff_merge_possible?
-      @status = :failed
-      return
-    end
-
     @merge_request.update(merge_error: nil)
 
     if params[:merge_when_build_succeeds].present? && @merge_request.ci_commit && @merge_request.ci_commit.active?

app/models/merge_request.rb

@@ -255,7 +255,7 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def mergeable?
-    open? && !work_in_progress? && can_be_merged?
+    open? && !work_in_progress? && can_be_merged? && !must_be_rebased?
   end
 
   def gitlab_merge_status
@@ -567,6 +567,10 @@ class MergeRequest < ActiveRecord::Base
     target_sha == source_sha_parent
   end
 
+  def must_be_rebased?
+    self.project.ff_merge_must_be_possible? && !ff_merge_possible?
+  end
+
   def rebase_dir_path
     File.join(Gitlab.config.shared.path, 'tmp/rebase', source_project.id.to_s, id.to_s).to_s
   end

app/services/merge_requests/ff_merge_service.rb

@@ -9,8 +9,6 @@ module MergeRequests
     private
 
     def commit
-      return false unless @merge_request.ff_merge_possible?
-
       repository.ff_merge(current_user, merge_request.source_sha, merge_request.target_branch)
     end
   end

app/views/projects/merge_requests/widget/_open.html.haml

@@ -19,7 +19,7 @@
       = render 'projects/merge_requests/widget/open/merge_when_build_succeeds'
     - elsif !@merge_request.can_be_merged_by?(current_user)
       = render 'projects/merge_requests/widget/open/not_allowed'
-    - elsif @project.ff_merge_must_be_possible? && !@merge_request.ff_merge_possible?
+    - elsif @merge_request.must_be_rebased?
       = render 'projects/merge_requests/widget/open/rebase'
     - else
       = render 'projects/merge_requests/widget/open/accept'