list-objects: drop name_path entirely

list-objects: drop name_path entirely

In the previous commit, we left name_path as a thin wrapper
around a strbuf. This patch drops it entirely. As a result,
every show_object_fn callback needs to be adjusted. However,
none of their code needs to be changed at all, because the
only use was to pass it to path_name(), which now handles
the bare strbuf.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>