Performance improvements for pull request list page (#29900)

This PR will avoid load pullrequest.Issue twice in pull request list
page. It will reduce x times database queries for those WIP pull
requests.

Partially fix #29585

---------

Co-authored-by: Giteabot <teabot@gitea.io>

models/issues/issue_list.go

@@ -370,6 +370,9 @@ func (issues IssueList) LoadPullRequests(ctx context.Context) error {
 
 	for _, issue := range issues {
 		issue.PullRequest = pullRequestMaps[issue.ID]
+		if issue.PullRequest != nil {
+			issue.PullRequest.Issue = issue
+		}
 	}
 	return nil
 }