builtin/maintenance: introduce "geometric" strategy

We have two different repacking strategies in Git:

  - The "gc" strategy uses git-gc(1).

  - The "incremental" strategy uses multi-pack indices and `git
    multi-pack-index repack` to merge together smaller packfiles as
    determined by a specific batch size.

The former strategy is our old and trusted default, whereas the latter
has historically been used for our scheduled maintenance. But both
strategies have their shortcomings:

  - The "gc" strategy performs regular all-into-one repacks. Furthermore
    it is rather inflexible, as it is not easily possible for a user to
    enable or disable specific subtasks.

  - The "incremental" strategy is not a full replacement for the "gc"
    strategy as it doesn't know to prune stale data.

So today, we don't have a strategy that is well-suited for large repos
while being a full replacement for the "gc" strategy.

Introduce a new "geometric" strategy that aims to fill this gap. This
strategy invokes all the usual cleanup tasks that git-gc(1) does like
pruning reflogs and rerere caches as well as stale worktrees. But where
it differs from both the "gc" and "incremental" strategy is that it uses
our geometric repacking infrastructure exposed by git-repack(1) to
repack packfiles. The advantage of geometric repacking is that we only
need to perform an all-into-one repack when the object count in a repo
has grown significantly.

One downside of this strategy is that pruning of unreferenced objects is
not going to happen regularly anymore. Every geometric repack knows to
soak up all loose objects regardless of their reachability, and merging
two or more packs doesn't consider reachability, either. Consequently,
the number of unreachable objects will grow over time.

This is remedied by doing an all-into-one repack instead of a geometric
repack whenever we determine that the geometric repack would end up
merging all packfiles anyway. This all-into-one repack then performs our
usual reachability checks and writes unreachable objects into a cruft
pack. As cruft packs won't ever be merged during geometric repacks we
can thus phase out these objects over time.

Of course, this still means that we retain unreachable objects for far
longer than with the "gc" strategy. But the maintenance strategy is
intended especially for large repositories, where the basic assumption
is that the set of unreachable objects will be significantly dwarfed by
the number of reachable objects.

If this assumption is ever proven to be too disadvantageous we could for
example introduce a time-based strategy: if the largest packfile has not
been touched for longer than $T, we perform an all-into-one repack. But
for now, such a mechanism is deferred into the future as it is not clear
yet whether it is needed in the first place.

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

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