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>

diff --cc tests/nitls.args
index e79a1e1,d87add0..5f5a31a
--- 1/tests/nitls.args
--- 2/tests/nitls.args
+++ b/tests/nitls.args
@@@ -3,6 -3,4 +3,6 @@@ base_simple3.nit project
  -t -r base_simple3.nit project1
  -s base_simple3.nit project1
  -M base_simple3.nit base_simple_import.nit
- -td project1/module3.nit
+ -td project1/module3.nit project1/subdir/subdir2/subdir3/submodule.nit
 +test_prog --no-color
 +test_prog/game/excluded.nit test_prog/game/excluded_dir/more.nit -t --no-color