am: pass exclude down to apply

am: pass exclude down to apply

This allows to pass patches around from repositories,
where the other repository doesn't feature certain files.

In the special case this works for dash git sync to klibc dash:
 git am --directory="usr/dash" --exclude="usr/dash/configure.ac" \
        --exclude="usr/dash/ChangeLog" --exclude="usr/dash/dash.1" \
	.. -i -s -k ../dash/000X-foo.patch

Signed-off-by: maximilian attems <max@stro.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>