log_tree_diff: get rid of extra check for NULL

Get rid of needless check of 'parents' for NULL. The NULL case
is already handled right above, and 'parents' is dereferenced
without check below anyway.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

log-tree.c

@@ -917,7 +917,7 @@ static int log_tree_diff(struct rev_info *opt, struct commit *commit, struct log
 	}
 
 	/* More than one parent? */
-	if (parents && parents->next) {
+	if (parents->next) {
 		if (opt->ignore_merges)
 			return 0;
 		else if (opt->combine_merges)