Check if MR is not broken.

app/models/merge_request.rb

@@ -254,7 +254,7 @@ class MergeRequest < ActiveRecord::Base
   end
   def mergeable?
-    return false unless open? && !work_in_progress?
+    return false unless open? && !work_in_progress? && !broken?
     check_if_can_be_merged