MyFirstObjectWalk: fix filtered object walk

Commit f0d2f84 (MyFirstObjectWalk: update recommended usage,
2022-03-09) changed a call of parse_list_objects_filter() in a way
that probably never worked: parse_list_objects_filter() always needed
a pointer as its first argument.

Fix this by removing the CALLOC_ARRAY and passing the address of
rev->filter to parse_list_objects_filter() in accordance to
such a call in revisions.c, for example.

Signed-off-by: Dirk Gouders <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: dgouders

Documentation/MyFirstObjectWalk.txt

@@ -734,8 +734,8 @@ walk we've just performed:
 	} else {
 		trace_printf(
 			_("Filtered object walk with filterspec 'tree:1'.\n"));
-		CALLOC_ARRAY(rev->filter, 1);
-		parse_list_objects_filter(rev->filter, "tree:1");
+
+		parse_list_objects_filter(&rev->filter, "tree:1");
 	}
 	traverse_commit_list(rev, walken_show_commit,
 			     walken_show_object, NULL);