Move the packfile verification out of `cmd_fsck()` and into the "packed"
source. While doing so, thread the progress meter and object callback
through the newly introduced `struct odb_fsck_options` so that the
caller's preferences are honoured without exposing those details at the
"builtin/fsck.c" level.
Note that the old code reported failures when verifying packfiles with
the `ERROR_PACK` bit, which gets returned to the caller via the exit
code. This bit is neither exercised in our test suite nor is it
documented anywhere in our codebase. Furthermore, this bit is highly
specific to the object storage backend, which makes it a bad fit for the
new pluggable infrastructure. So instead of retaining these semantics,
we drop them and return the generic `ERROR_OBJECT` bit.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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