Calculating duration from a single uint64_t is simpler than from a struct
timeval. Change performance measurement for 'advice.statusuoption' from
gettimeofday() to getnanotime().
Also initialize t_begin to prevent uninitialized variable warning.
Signed-off-by: Karsten Blees <blees@dcon.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>