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>

pretty.c

@@ -956,9 +956,8 @@ static size_t format_commit_one(struct strbuf *sb, const char *placeholder,
 			if (c->pretty_ctx->reflog_info)
 				get_reflog_selector(sb,
 						    c->pretty_ctx->reflog_info,
-						    c->pretty_ctx->date_mode_explicit ?
-						      c->pretty_ctx->date_mode :
-						      DATE_NORMAL,
+						    c->pretty_ctx->date_mode,
+						    c->pretty_ctx->date_mode_explicit,
 						    (placeholder[1] == 'd'));
 			return 2;
 		case 's':	/* reflog message */