packfile: track packs via the MRU list exclusively

We track packfiles via two different lists:

  - `struct packfile_store::packs` is a list that sorts local packs
    first. In addition, these packs are sorted so that younger packs are
    sorted towards the front.

  - `struct packfile_store::mru` is a list that sorts packs so that
    most-recently used packs are at the front.

The reasoning behind the ordering in the `packs` list is that younger
objects stored in the local object store tend to be accessed more
frequently, and that is certainly true for some cases. But there are
going to be lots of cases where that isn't true. Especially when
traversing history it is likely that one needs to access many older
objects, and due to our housekeeping it is very likely that almost all
of those older objects will be contained in one large pack that is
oldest.

So whether or not the ordering makes sense really depends on the use
case at hand. A flexible approach like our MRU list addresses that need,
as it will sort packs towards the front that are accessed all the time.
Intuitively, this approach is thus able to satisfy more use cases more
efficiently.

This reasoning casts some doubt on whether or not it really makes sense
to track packs via two different lists. It causes confusion, and it is
not clear whether there are use cases where the `packs` list really is
such an obvious choice.

Merge these two lists into one most-recently-used list.

Note that there is one important edge case: `for_each_packed_object()`
uses the MRU list to iterate through packs, and then it lists each
object in those packs. This would have the effect that we now sort the
current pack towards the front, thus modifying the list of packfiles we
are iterating over, with the consequence that we'll see an infinite
loop. This edge case is worked around by introducing a new field that
allows us to skip updating the MRU.

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

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