use commit_list_count() to count the members of commit_lists

Call commit_list_count() instead of open-coding it repeatedly.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/pretty.c b/pretty.c
index 6e54934..ac800fa 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1556,12 +1556,7 @@
 		}
 
 		if (!parents_shown) {
-			struct commit_list *parent;
-			int num;
-			for (parent = commit->parents, num = 0;
-			     parent;
-			     parent = parent->next, num++)
-				;
+			unsigned num = commit_list_count(commit->parents);
 			/* with enough slop */
 			strbuf_grow(sb, num * 50 + 20);
 			add_merge_info(pp, sb, commit);