ref-filter: drop unused buf/sz pairs

ref-filter: drop unused buf/sz pairs

The grab_tag_values() and grab_commit_values() functions take both the
"struct object" as well as the buf/sz pair for the actual object bytes.
However, neither function uses the latter, as they pull the data
directly from the parsed object struct.

Let's get rid of these misleading parameters.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>