dir.c: fix ignore processing within not-ignored directories

As of 95c6f271 "dir.c: unify is_excluded and is_path_excluded APIs", the
is_excluded API no longer recurses into directories that match an ignore
pattern, and returns the directory's ignored state for all contained paths.

This is OK for normal ignore patterns, i.e. ignoring a directory affects
the entire contents recursively.

Unfortunately, this also "works" for negated ignore patterns ('!dir'), i.e.
the entire contents is "not-ignored" recursively, regardless of ignore
patterns that match the contents directly.

In prep_exclude, skip recursing into a directory only if it is really
ignored (i.e. the ignore pattern is not negated).

Signed-off-by: Karsten Blees <blees@dcon.de>
Tested-by: Øystein Walle
Signed-off-by: Junio C Hamano <gitster@pobox.com>