log: convert to parse-options

log: convert to parse-options

Use parse-options in cmd_log_init instead of manually iterating
through them. This makes the code a bit cleaner but more importantly
allows us to catch the "--quiet" option which causes some of the
log-related commands to misbehave as it would otherwise get passed on
to the diff.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>