fixed z-index issue with merge request versions widget

app/assets/stylesheets/pages/merge_requests.scss

@@ -691,8 +691,10 @@
 }
 .mr-version-controls {
+  position: relative;
   background: $gray-light;
   color: $gl-text-color;
+  z-index: 199;
   .mr-version-menus-container {
     display: -webkit-flex;