dir.c: fix EXC_FLAG_MUSTBEDIR match in sparse checkout

Commit c84de707 (excluded_1(): support exclude files in index -
2009-08-20) tries to work around the fact that there is no
directory/file information in index entries, therefore
EXC_FLAG_MUSTBEDIR match would fail.

Unfortunately the workaround is flawed. This fixes it.

Reported-by: Thomas Rinderknecht <thomasr@sailguy.org>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>