Merge: nitls: fix tree representation of nested groups

Merge: nitls: fix tree representation of nested groups

nitls wrongly detached some nested groups in tree-mode.

Before:

~~~
$ nitls -tp lib/nitcorn/examples/src/nitcorn_hello_world.nit lib/nitcorn/http_response.nit
lib/nitcorn/examples
`--lib/nitcorn/examples/src
   `--lib/nitcorn/examples/src/nitcorn_hello_world.nit
lib/nitcorn
`--lib/nitcorn/http_response.nit
~~~

After:

~~~
$ nitls -tp lib/nitcorn/examples/src/nitcorn_hello_world.nit lib/nitcorn/http_response.nit
lib/nitcorn
|--lib/nitcorn/examples
|  `--lib/nitcorn/examples/src
|     `--lib/nitcorn/examples/src/nitcorn_hello_world.nit
`--lib/nitcorn/http_response.nit
~~~

Pull-Request: #2161
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>