reflog-walk: tell explicit --date=default from not having --date at all

reflog-walk: tell explicit --date=default from not having --date at all

Introduction of opt->date_mode_explicit was a step in the right direction,
but lost that crucial bit at the very end of the callchain, and the callee
could not tell an explicitly specified "I want *date* but in default format"
from the built-in default value passed when there was no --date specified.

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