MyFirstObjectWalk: fix filtered object walk

MyFirstObjectWalk: fix filtered object walk

Commit f0d2f84919 (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 <redacted>
Signed-off-by: Junio C Hamano <redacted>