Enable MergeableSelector in scss-lint (for !14398)

app/assets/stylesheets/pages/notes.scss

@@ -715,9 +715,8 @@ ul.notes {
     svg path {
       fill: $gray-darkest;
     }
-  }
 
-  .btn.discussion-create-issue-btn {
+    &.discussion-create-issue-btn {
       margin-left: -4px;
       border-radius: 0;
       border-right: 0;
@@ -732,6 +731,7 @@ ul.notes {
         }
       }
     }
+  }
 }
 
 .line-resolve-all {