'struct cache' does not have a 'usec' member, but a 'unsigned int
nsec' member. Simmilar 'struct stat' does not have a 'st_mtim.usec'
member, and we should instead use 'st_mtim.tv_nsec'.
Signed-off-by: Kjetil Barvik <barvik@broadpark.no> Signed-off-by: Junio C Hamano <gitster@pobox.com>