refs/packed: de-globalize handling of "core.packedRefsTimeout"

refs/packed: de-globalize handling of "core.packedRefsTimeout"

When locking the "packed-refs" file we allow the user to configure a
timeout for how long we try taking the lock. This is configurable via
"core.packedRefsTimeout", which we parse in `packed_refs_lock()`.

The parsed value is stored in function-static variables though, which of
course has the effect that we'll only ever use the timeout configured in
the first packed reference store that we see. Consequently, if we ever
were to handle stores from different repositories, then we'd use the
same configuration for both stores even if they diverge.

This is of course a somewhat theoretical concern -- we don't typically
handle multiple packed stores, and even if we did it's very unlikely
that the user has configured different timeout values for each of them.
But still, this is a code smell, and an unnecessary one, too.

Fix the issue by moving the value into `struct packed_ref_store` so that
it can be parsed per store.

This removes the last callsite that still used `the_repository`, so drop
the `USE_THE_REPOSITORY_VARIABLE` define.

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