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>

builtin-fetch-pack.c

@@ -807,7 +807,7 @@ struct ref *fetch_pack(struct fetch_pack_args *my_args,
 				die("shallow file was removed during fetch");
 		} else if (st.st_mtime != mtime.sec
 #ifdef USE_NSEC
-				|| st.st_mtim.tv_nsec != mtime.nsec
+				|| ST_CTIME_NSEC(st) != mtime.nsec
 #endif
 			  )
 			die("shallow file was changed during fetch");