read_index_from(): catch out of order entries when reading an index file

Signed-off-by: Jaime Soriano Pastor <jsorianopastor@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
read_index_unmerged(): remove unnecessary loop index adjustment

Signed-off-by: Jaime Soriano Pastor <jsorianopastor@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>