Performance improvements for pull request list page (#29900) (#29972)

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
Backport #29900

routers/api/v1/repo/issue.go

@@ -864,10 +864,11 @@ func EditIssue(ctx *context.APIContext) {
 	}
 	if form.State != nil {
 		if issue.IsPull {
-			if pr, err := issue.GetPullRequest(); err != nil {
+			if err := issue.LoadPullRequest(ctx); err != nil {
 				ctx.Error(http.StatusInternalServerError, "GetPullRequest", err)
 				return
-			} else if pr.HasMerged {
+			}
+			if issue.PullRequest.HasMerged {
 				ctx.Error(http.StatusPreconditionFailed, "MergedPRState", "cannot change state of this pull request, it was already merged")
 				return
 			}