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

fix #38506

* 1.26: the "show form" button and "submit form" button are all red if
some checks fail
* 1.27.0: the "show form" button uses primary color, while "submit form"
button is red if some checks fail
* this fix: revert to 1.26

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';
 });