builtin/gc: fix crash when running git maintenance start (#5198)

> This patch was sent upstream by Patrick. I'm contributing it to Git
for Windows quickly to make sure it gets into microsoft/git, but also in
advance of any potential 2.47.1.

It was reported on the mailing list that running `git maintenance start`
immediately segfaults starting with b6c3f8e (builtin/maintenance: fix
leak in `get_schedule_cmd()`, 2024-09-26). And indeed, this segfault is
trivial to reproduce up to a point where one is scratching their head
why we didn't catch this regression in our test suite.

The root cause of this error is `get_schedule_cmd()`, which does not
populate the `out` parameter in all cases anymore starting with the
mentioned commit. Callers do assume it to always be populated though and
will e.g. call `strvec_split()` on the returned value, which will of
course segfault when the variable is uninitialized.

So why didn't we catch this trivial regression? The reason is that our
tests always set up the "GIT_TEST_MAINT_SCHEDULER" environment variable
via "t/test-lib.sh", which allows us to override the scheduler command
with a custom one so that we don't accidentally modify the developer's
system. But the faulty code where we don't set the `out` parameter will
only get hit in case that environment variable is _not_ set, which is
never the case when executing our tests.

Fix the regression by again unconditionally allocating the value in the
`out` parameter, if provided. Add a test that unsets the environment
variable to catch future regressions in this area.

Author: dscho

t/t7900-maintenance.sh

@@ -646,7 +646,7 @@ test_expect_success !MINGW 'register and unregister with regex metacharacters' '
 		maintenance.repo "$(pwd)/$META"
 '
 
-test_expect_success 'start without GIT_TEST_MAINT_SCHEDULER' '
+test_expect_success !MINGW,!DARWIN 'start without GIT_TEST_MAINT_SCHEDULER' '
 	test_when_finished "rm -rf crontab.log script repo" &&
 	mkdir script &&
 	write_script script/crontab <<-EOF &&

t/test-lib.sh

@@ -1738,6 +1738,12 @@ case $uname_s in
 	test_set_prereq GREP_STRIPS_CR
 	test_set_prereq WINDOWS
 	;;
+*Darwin*)
+	test_set_prereq POSIXPERM
+	test_set_prereq BSLASHPSPEC
+	test_set_prereq EXECKEEPSPID
+	test_set_prereq DARWIN
+	;;
 *)
 	test_set_prereq POSIXPERM
 	test_set_prereq BSLASHPSPEC