t7900: fix flaky "maintenance.strategy" test

One of our tests for whether "maintenance.strategy" is being respected
in t7900 is flaky in our CI systems:

    + GIT_TRACE2_EVENT=/tmp/test-output/trash directory.t7900-maintenance/repo/trace2.txt git -c maintenance.strategy=incremental maintenance run --quiet
    + test_maintenance_tasks trace2.txt
    + cat
    + sed -ne s/.*"region_enter".*"category":"maintenance\([^"]*\)".*"label":"\([^"][^"]*\)".*/\2\1/p trace2.txt
    + test_cmp expect actual
    + test 2 -ne 2
    + eval /usr/bin/diff -u "$@"
    + /usr/bin/diff -u expect actual
    --- expect	2026-08-07 06:20:51.388322602 +0000
    +++ actual	2026-08-07 06:20:51.388322602 +0000
    @@ -1,2 +0,0 @@
    -gc foreground
    -gc

When running with the "incremental" strategy, we expect two git-gc(1)
tasks to have been executed, but sometimes the test simply doesn't
execute any of those tasks.

A first hunch may be that maybe the disk-state is sometimes different
and thus we decide not to run maintenance. But git-maintenance(1)
doesn't run with the "--auto" switch, so we should execute those tasks
regardless of the on-disk state.

But there's a second condition that may cause us to not execute tasks,
namely when the "maintenance.lock" file exists due to a concurrently
running git-maintenance(1) process. We usually disable auto-maintenance
from detaching in our test suite to avoid exactly these kinds of race
conditions by exporting `GIT_TEST_MAINT_AUTO_DETACH=false`. But in t7900
we unset "GIT_TEST_MAINT_AUTO_DETACH" and thus enable the auto-detach
logic. The intent of this is to exercise git-maintenance(1) closer to
how it would run in a real-world scenario, but it does cause us to race
when the detached maintenance job that was triggered by `test_commit()`
lives long enough.

We could trivially fix this race by disabling auto-maintenance for this
specific test. But that doesn't fix this class of races in this test
suite: while I haven't seen any of the other tests fail in the same way,
a bunch of them have this race, as well.

Instead, let's retain "GIT_TEST_MAINT_AUTO_DETACH" and only unset it as
required.

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

Author: pks-t

t/t7900-maintenance.sh

@@ -7,9 +7,6 @@ test_description='git maintenance builtin'
 GIT_TEST_COMMIT_GRAPH=0
 GIT_TEST_MULTI_PACK_INDEX=0
 
-# Ensure that auto-maintenance detaches as usual.
-sane_unset GIT_TEST_MAINT_AUTO_DETACH
-
 test_lazy_prereq XMLLINT '
 	xmllint --version
 '
@@ -65,6 +62,7 @@ test_expect_success 'maintenance.auto config option' '
 	git init repo &&
 	(
 		cd repo &&
+		sane_unset GIT_TEST_MAINT_AUTO_DETACH &&
 
 		GIT_TRACE2_EVENT="$(pwd)/default" git commit --quiet --allow-empty -m 1 &&
 		test_subcommand git maintenance run --auto --quiet --detach <default &&
@@ -84,6 +82,7 @@ test_expect_success 'gc.auto config option' '
 	git init repo &&
 	(
 		cd repo &&
+		sane_unset GIT_TEST_MAINT_AUTO_DETACH &&
 
 		GIT_TRACE2_EVENT="$(pwd)/default" git commit --quiet --allow-empty -m 1 &&
 		test_subcommand git maintenance run --auto --quiet --detach <default &&
@@ -101,6 +100,7 @@ test_expect_success 'maintenance.auto overrides gc.auto' '
 	git init repo &&
 	(
 		cd repo &&
+		sane_unset GIT_TEST_MAINT_AUTO_DETACH &&
 
 		git config set maintenance.auto false &&
 		git config set gc.auto 1 &&