fix: make the merge box button red if some checks fail (#38508) (#38516)

Backport #38508

fix #38506

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/components/PullRequestMergeForm.vue

@@ -39,6 +39,7 @@ const mergeButtonStyleClass = computed(() => {
 const mergeSelectStyleClass = computed(() => {
   if (mergeForm.emptyCommit) return '';
   if (mergeStyle.value === mergeStyleManuallyMerged) return 'red';
+  if (!mergeForm.allOverridableChecksOk) return 'red';
   return 'primary';
 });