t: fix races caused by background maintenance

Many Git commands spawn git-maintenance(1) to optimize the repository in
the background. By default, performing the maintenance is for most of
the part asynchronous: we fork the executable and then continue with the
rest of our business logic.

This is working as expected for our users, but this behaviour is
somewhat problematic for our test suite as this is inherently racy. We
have many tests that verify the on-disk state of repositories, and those
tests may easily race with our background maintenance. In a similar
fashion, we may end up with processes that "leak" out of a current test
case.

Until now this tends to not be much of a problem. Our maintenance uses
git-gc(1) by default, which knows to bail out in case there aren't
either too many packfiles or too many loose objects. So even if other
data structures would need to be optimized, we won't do so unless the
object database also needs optimizations.

This is about to change though, as a subsequent commit will switch to
the "geometric" maintenance strategy as a default. The consequence is
that we will run required optimizations even if the object database is
well-optimized. And this uncovers races between our test suite and
background maintenance all over the place.

Disabling maintenance outright in our test suite is not really an
option, as it would result in significant divergence from the "real
world" and reduce our test coverage. But we've got an alternative up our
sleeves: we can ensure that garbage collection runs synchronously by
overriding the "maintenance.autoDetach" configuration.

Of course that also diverges from the real world, as we now stop testing
that background maintenance interacts in a benign way with normal Git
commands. But on the other hand this ensures that the maintenance itself
does not for example lead to data loss in a more reproducible way.

Another concern is that this would make execution of the test suite much
slower. But a quick benchmark on my machine demonstrates that this does
not seem to be the case:

    Benchmark 1: meson test (revision = HEAD~)
      Time (mean ± σ):     131.182 s ±  1.293 s    [User: 853.737 s, System: 1160.479 s]
      Range (min … max):   130.001 s … 132.563 s    3 runs

    Benchmark 2: meson test (revision = HEAD)
      Time (mean ± σ):     129.554 s ±  0.507 s    [User: 849.040 s, System: 1152.664 s]
      Range (min … max):   129.000 s … 129.994 s    3 runs

    Summary
      meson test (revision = HEAD) ran
        1.01 ± 0.01 times faster than meson test (revision = HEAD~)

Funny enough, it even seems as if this speeds up test execution ever so
slightly, but that may just as well be noise.

Introduce a new `GIT_TEST_MAINT_AUTO_DETACH` environment variable that
allows us to override the auto-detach behaviour and set that variable in
our tests.

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

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