bulk-checkin: use repository variable from transaction

bulk-checkin: use repository variable from transaction

The bulk-checkin subsystem depends on `the_repository`. Adapt functions
and call sites to access the repository through `struct odb_transaction`
instead. The `USE_THE_REPOSITORY_VARIBALE` is still required as the
`pack_compression_level` and `pack_size_limit_cfg` globals are still
used.

Also adapt functions using packfile state to instead access it through
the transaction. This makes some function parameters redundant and go
away.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>