builtin/fsck: stop using `the_repository` when checking packed objects

builtin/fsck: stop using `the_repository` when checking packed objects

We implicitly rely on `the_repository` when checking objects part of a
packfile. These objects are iterated over via `verify_pack()`, which is
provided by the packfile subsystem, and a callback function is then
invoked for each of the objects in that specific pack.

Unfortunately, it is not possible to provide a payload to the callback
function. Refactor `verify_pack()` to accept a payload that is passed
through to the callback so that we can inject the repository and get rid
of the use of `the_repository`.

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