packfile: drop repository parameter from `packed_object_info()`

packfile: drop repository parameter from `packed_object_info()`

The function `packed_object_info()` takes a packfile and offset and
returns the object info for the corresponding object. Despite these two
parameters though it also takes a repository pointer. This is redundant
information though, as `struct packed_git` already has a repository
pointer that is always populated.

Drop the redundant parameter.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>