t1300: fix unset of GIT_CONFIG_NOSYSTEM leaking into subsequent tests

In order to test whether the new GIT_CONFIG_SYSTEM environment variable
behaves as expected, we unset GIT_CONFIG_NOSYSTEM in one of our tests in
t1300. But because tests are not executed in a subshell, this unset
leaks into all subsequent tests and may thus cause them to fail in some
environments. These failures are easily reproducable with `make
prefix=/root test`.

Fix the issue by not using `sane_unset GIT_CONFIG_NOSYSTEM`, but instead
just manually add it to the environment of the two command invocations
which need it.

Reported-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1300-config.sh

@@ -2092,21 +2092,20 @@ test_expect_success 'override global and system config' '
 	git config --show-scope --list >output &&
 	test_cmp expect output &&
 
-	sane_unset GIT_CONFIG_NOSYSTEM &&
-
 	cat >expect <<-EOF &&
 	system	system.config=true
 	global	global.config=true
 	local	local.config=true
 	EOF
-	GIT_CONFIG_SYSTEM=custom-system-config GIT_CONFIG_GLOBAL=custom-global-config \
+	GIT_CONFIG_NOSYSTEM=false GIT_CONFIG_SYSTEM=custom-system-config GIT_CONFIG_GLOBAL=custom-global-config \
 		git config --show-scope --list >output &&
 	test_cmp expect output &&
 
 	cat >expect <<-EOF &&
 	local	local.config=true
 	EOF
-	GIT_CONFIG_SYSTEM=/dev/null GIT_CONFIG_GLOBAL=/dev/null git config --show-scope --list >output &&
+	GIT_CONFIG_NOSYSTEM=false GIT_CONFIG_SYSTEM=/dev/null GIT_CONFIG_GLOBAL=/dev/null \
+		git config --show-scope --list >output &&
 	test_cmp expect output
 '