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>

tests/integration/actions_trigger_test.go

@@ -1801,7 +1801,7 @@ jobs:
 		testEditFile(t, session, "user2", repoName, repo.DefaultBranch, "dir1/dir1.txt", "11")
 		// update by rebase
 		req := NewRequest(t, "POST", fmt.Sprintf("/%s/%s/pulls/%d/update?style=rebase", "user2", repoName, apiPull.Index))
-		session.MakeRequest(t, req, http.StatusSeeOther)
+		session.MakeRequest(t, req, http.StatusOK)
 		runner.fetchNoTask(t)
 	})
 }

tests/integration/issue_test.go

@@ -692,9 +692,9 @@ func TestIssueReferenceURL(t *testing.T) {
 	htmlDoc := NewHTMLParser(t, resp.Body)
 
 	// the "reference" uses relative URLs, then JS code will convert them to absolute URLs for current origin, in case users are using multiple domains
-	ref, _ := htmlDoc.Find(`.timeline-item.comment.first .reference-issue`).Attr("data-reference")
+	ref, _ := htmlDoc.Find(`.timeline-item.comment.issue-content-comment .reference-issue`).Attr("data-reference")
 	assert.Equal(t, "/user2/repo1/issues/1#issue-1", ref)
 
-	ref, _ = htmlDoc.Find(`.timeline-item.comment:not(.first) .reference-issue`).Attr("data-reference")
+	ref, _ = htmlDoc.Find(`.timeline-item.comment:not(.issue-content-comment) .reference-issue`).Attr("data-reference")
 	assert.Equal(t, "/user2/repo1/issues/1#issuecomment-2", ref)
 }