packfile: pass source to `prepare_pack()`

packfile: pass source to `prepare_pack()`

When preparing a packfile we pass various pieces attached to the pack's
object database source via the `struct prepare_pack_data`. Refactor this
code to instead pass in the source directly. This reduces the number of
variables we need to pass and allows for a subsequent refactoring where
we start to prepare the pack via the source.

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