Order pull request conflict checking by recently updated, for each push (#23220)

When a change is pushed to the default branch and many pull requests are
open for that branch, conflict checking can take some time.

Previously it would go from oldest to newest pull request. Now
prioritize pull requests that are likely being actively worked on or
prepared for merging.

This only changes the order within one push to one repository, but the
change is trivial and can already be quite helpful for smaller Gitea
instances where a few repositories have most pull requests. A global
order would require deeper changes to queues.

models/issues/pull_list.go

@@ -111,6 +111,7 @@ func GetUnmergedPullRequestsByBaseInfo(repoID int64, branch string) ([]*PullRequ
 	return prs, db.GetEngine(db.DefaultContext).
 		Where("base_repo_id=? AND base_branch=? AND has_merged=? AND issue.is_closed=?",
 			repoID, branch, false, false).
+		OrderBy("issue.updated_unix DESC").
 		Join("INNER", "issue", "issue.id=pull_request.issue_id").
 		Find(&prs)
 }