Not all systems use st_[cm]tim field for ns resolution file timestamp

Not all systems use st_[cm]tim field for ns resolution file timestamp

Some codepaths do not still use the ST_[CM]TIME_NSEC() pair of macros
introduced by the previous commit but assumes all systems use st_mtim
and st_ctim fields in "struct stat" to record nanosecond resolution part
of the file timestamps.

Signed-off-by: Junio C Hamano <gitster@pobox.com>