root目前只支持绝对地址吗？

如下：
```
location /img/ {
root ../images;
concat on;
concat_max_files 20;
}
```
这样配置下，只能找到普通静态请求，二合并静态资源请求都不行
是concat不支持吗？

Reproduce the shown nginx location with a relative root and compare ordinary static requests with concat requests. Trace the concat request handling and root-path resolution, then verify that relative roots behave consistently or that the configuration limitation is documented and covered by a regression test.

Written by the indexing model from the issue text.