status: always report ignored tracked directories

Tracked directories (i.e. directories containing tracked files) that
are ignored must be reported as ignored if they contain untracked files.

Currently, files in the index can't be reported as ignored and are
automatically dropped from the list:

 - When core.ignorecase is false, directories (which are not directly
 tracked) are not listed as part of the index, and the directory can be
 shown as ignored.

 - When core.ignorecase is true on the other hand, directories are
 reported as part of the index, and the directory is dropped, thus not
 displayed as ignored.

Fix that behavior by allowing indexed file to be added when looking for
ignored files.

 - Ignored tracked and untracked directories are treated the same way
 when looking for ignored files, so we should not care about their index
 status.
 - Files are dismissed by treat_file() if they belong to the
 index, so that step would have been a no-op anyway.

Signed-off-by: Antoine Pelisse <apelisse@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>