log --grep-reflog: reject the option without -g

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

grep.c

@@ -64,6 +64,8 @@ void append_header_grep_pattern(struct grep_opt *opt,
 {
 	struct grep_pat *p = create_grep_pat(pat, strlen(pat), "header", 0,
 					     GREP_PATTERN_HEAD, field);
+	if (field == GREP_HEADER_REFLOG)
+		opt->use_reflog_filter = 1;
 	do_append_grep_pat(&opt->header_tail, p);
 }