Fix various problems (#37547)

1. Fix ugly commit form "warning" message
2. Use JSONError for "Update PR Branch" response 
3. Remove useless "timeline" class
4. Make timeline review default to "comment" to avoid icon missing
5. Align PR's "command line instructions" UI
6. Simply "Update PR branch" button logic

And then some TODOs are fixed.

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/features/repo-issue-content.ts

@@ -94,7 +94,7 @@ function showContentHistoryDetail(issueBaseUrl: string, commentId: string, histo
 function showContentHistoryMenu(issueBaseUrl: string, elCommentItem: Element, commentId: string) {
   const elHeaderLeft = elCommentItem.querySelector('.comment-header-left')!;
   const menuHtml = `
-  <div class="ui dropdown interact-fg content-history-menu" data-comment-id="${commentId}">
+  <div class="ui dropdown interact-fg content-history-menu tw-flex-shrink-0" data-comment-id="${commentId}">
     &bull; ${i18nTextEdited}${svg('octicon-triangle-down', 14, 'dropdown icon')}
     <div class="menu">
     </div>
@@ -127,7 +127,7 @@ export async function initRepoIssueContentHistory() {
   const issuePageInfo = parseIssuePageInfo();
   if (!issuePageInfo.issueNumber) return;
 
-  const elIssueDescription = document.querySelector('.repository.issue .timeline-item.comment.first'); // issue(PR) main content
+  const elIssueDescription = document.querySelector('.repository.issue .timeline-item.comment.issue-content-comment'); // issue(PR) main content
   const elComments = document.querySelectorAll('.repository.issue .comment-list .comment'); // includes: issue(PR) comments, review comments, code comments
   if (!elIssueDescription && !elComments.length) return;