Make target branch dropdown on merge request edit page consistent

app/assets/stylesheets/framework/selects.scss

@@ -280,13 +280,21 @@
 // TODO: change global style
 .ajax-project-dropdown,
 body[data-page="projects:new"] #select2-drop,
+body[data-page="projects:merge_requests:edit"] #select2-drop,
 body[data-page="projects:blob:new"] #select2-drop,
 body[data-page="profiles:show"] #select2-drop,
 body[data-page="projects:blob:edit"] #select2-drop {
   &.select2-drop {
+    border: 1px solid $dropdown-border-color;
+    border-radius: $border-radius-base;
     color: $gl-text-color;
   }
 
+  &.select2-drop-above {
+    border-top: none;
+    margin-top: -4px;
+  }
+
   .select2-results {
     .select2-no-results,
     .select2-searching,