builtin/maintenance: use "geometric" strategy by default

The git-gc(1) command has been introduced in the early days of Git in
30f610b7b0 (Create 'git gc' to perform common maintenance operations.,
2006-12-27) as the main repository maintenance utility. And while the
tool has of course evolved since then to cover new parts, the basic
strategy it uses has never really changed much.

It is safe to say that since 2006 the Git ecosystem has changed quite a
bit. Repositories tend to be much larger nowadays than they have been
almost 20 years ago, and large parts of the industry went crazy for
monorepos (for various wildly different definitions of "monorepo"). So
the maintenance strategy we used back then may not be the best fit
nowadays anymore.

Arguably, most of the maintenance tasks that git-gc(1) does are still
perfectly fine today: repacking references, expiring various data
structures and things like tend to not cause huge problems. But the big
exception is the way we repack objects.

git-gc(1) by default uses a split strategy: it performs incremental
repacks by default, and then whenever we have too many packs we perform
a large all-into-one repack. This all-into-one repack is what is causing
problems nowadays, as it is an operation that is quite expensive. While
it is wasteful in small- and medium-sized repositories, in large repos
it may even be prohibitively expensive.

We have eventually introduced git-maintenance(1) that was slated as a
replacement for git-gc(1). In contrast to git-gc(1), it is much more
flexible as it is structured around configurable tasks and strategies.
So while its default "gc" strategy still uses git-gc(1) under the hood,
it allows us to iterate.

A second strategy it knows about is the "incremental" strategy, which we
configure when registering a repository for scheduled maintenance. This
strategy isn't really a full replacement for git-gc(1) though, as it
doesn't know to expire unused data structures. In Git 2.52 we have thus
introduced a new "geometric" strategy that is a proper replacement for
the old git-gc(1).

In contrast to the incremental/all-into-one split used by git-gc(1), the
new "geometric" strategy maintains a geometric progression of packfiles,
which significantly reduces the number of all-into-one repacks that we
have to perform in large repositories. It is thus a much better fit for
large repositories than git-gc(1).

Note that the "geometric" strategy isn't perfect though: while we
perform way less all-into-one repacks compared to git-gc(1), we still
have to perform them eventually. But for the largest repositories out
there this may not be an option either, as client machines might not be
powerful enough to perform such a repack in the first place. These cases
would thus still be covered by the "incremental" strategy.

Switch the default strategy away from "gc" to "geometric", but retain
the "incremental" strategy configured when registering background
maintenance with `git maintenance register`.

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

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