Fix: no longer shows folders recursively to root

In the previous implementation, Jedi would's traverse_parents function
traversed parent directories to the system root every time. This would
inadvertently add every folder to the system root every time. Obviously,
this is not the behavior desired for the import system.

This pull request provides a new argument to the traverse_parents
function, "root", which represents the root parent for the search. This
argument defaults to None, thereby preserving the existing behavior of
the function.

I chose to duplicate some code for performance reasons. Since I'm trying
to avoid too much path manipulation magic, we do:

* a search to a valid specified root, OR
* a simple upward search until hitting the system root when there is no
valid root specified.

jedi/api/project.py

@@ -110,7 +110,10 @@ class Project(object):
                 suffixed += discover_buildout_paths(inference_state, inference_state.script_path)
 
                 if add_parent_paths:
-                    traversed = list(traverse_parents(inference_state.script_path))
+                    traversed = list(traverse_parents(
+                        inference_state.script_path,
+                        root=self._path,
+                    ))
 
                     # AFAIK some libraries have imports like `foo.foo.bar`, which
                     # leads to the conclusion to by default prefer longer paths