list-objects: pass callback data to show_objects()

list-objects: pass callback data to show_objects()

The traverse_commit_list() API takes two callback functions, one to show
commit objects, and the other to show other kinds of objects. Even though
the former has a callback data parameter, so that the callback does not
have to rely on global state, the latter does not.

Give the show_objects() callback the same callback data parameter.

Signed-off-by: Junio C Hamano <gitster@pobox.com>