treat_directory(): do not declare submodules to be untracked

treat_directory(): do not declare submodules to be untracked

When the working tree walker encounters a directory, it asks the
function treat_directory() if it should descend into it, show it as
an untracked directory, or do something else.  When the directory is
the top of the submodule working tree, we used to say "That is an
untracked directory", which was bogus.

It is an entity that is tracked in the index of the repository we
are looking at, and that is not to be descended into it.  Return
path_none, not path_untracked, to report that.

The existing case that path_untracked is returned for a newly
discovered submodule that is not tracked in the index (this only
happens when DIR_NO_GITLINKS option is not used) is unchanged, but
that is exactly because the submodule is not tracked in the index.

Signed-off-by: Junio C Hamano <gitster@pobox.com>