grep -An -Bm: fix invocation of external grep command

grep -An -Bm: fix invocation of external grep command

When building command line to invoke external grep, the
arguments to -A/-B/-C options were placd in randarg[] buffer,
but the code forgot that snprintf() does not count terminating
NUL in its return value.  This caused "git grep -A1 -B2" to
invoke external grep with "-B21 -A1".

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