Move print_commit_list to libgit.a

This is used by bisect.c, part of libgit.a while it stays in
builtin/rev-list.c. Move it to commit.c so that we won't get undefined
reference if a program that uses libgit.a happens to pull it in.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>

commit.c

@@ -1347,3 +1347,13 @@ struct commit_list **commit_list_append(struct commit *commit,
 	new->next = NULL;
 	return &new->next;
 }
+
+void print_commit_list(struct commit_list *list,
+		       const char *format_cur,
+		       const char *format_last)
+{
+	for ( ; list; list = list->next) {
+		const char *format = list->next ? format_cur : format_last;
+		printf(format, sha1_to_hex(list->item->object.sha1));
+	}
+}