builtin/receive-pack: convert to use git-maintenance(1)

builtin/receive-pack: convert to use git-maintenance(1)

In 850b6edefa (auto-gc: extract a reusable helper from "git fetch",
2020-05-06), we have introduced a helper function `run_auto_gc()` that
kicks off `git gc --auto`. The intent of this function was to pass down
the "--quiet" flag to git-gc(1) as required without duplicating this at
all callsites. In 7c3e9e8cfb (auto-gc: pass --quiet down from am,
commit, merge and rebase, 2020-05-06) we then converted callsites that
need to pass down this flag to use the new helper function. This has the
notable omission of git-receive-pack(1), which is the only remaining
user of `git gc --auto` that sets up the proccess manually. This is
probably because it unconditionally passes down the `--quiet` flag and
thus didn't benefit much from the new helper function.

In a95ce12430 (maintenance: replace run_auto_gc(), 2020-09-17) we then
replaced `run_auto_gc()` with `run_auto_maintenance()` which invokes
git-maintenance(1) instead of git-gc(1). This command is the modern
replacement for git-gc(1) and is both more thorough and also more
flexible because administrators can configure which tasks exactly to run
during maintenance.

But due to git-receive-pack(1) not using `run_auto_gc()` in the first
place it did not get converted to use git-maintenance(1) like we do
everywhere else now. Address this oversight and start to use the newly
introduced function `prepare_auto_maintenance()`. This will also make it
easier for us to adapt this code together with all the other callsites
that invoke auto-maintenance in the future.

This removes the last internal user of `git gc --auto`.

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