t0012: verify that built-ins handle -h even without gitdir

We just fixed a class of recently introduced bugs where calling, say,
`git fetch -h` outside a repository would not show the usage but instead
show an ugly `BUG` message.

Let's verify that this does not regress anymore.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

t/t0012-help.sh

@@ -139,13 +139,18 @@ test_expect_success 'git help --config-sections-for-completion' '
 '
 
 test_expect_success 'generate builtin list' '
+	mkdir -p sub &&
 	git --list-cmds=builtins >builtins
 '
 
 while read builtin
 do
 	test_expect_success "$builtin can handle -h" '
-		test_expect_code 129 git $builtin -h >output 2>&1 &&
+		(
+			GIT_CEILING_DIRECTORIES=$(pwd) &&
+			export GIT_CEILING_DIRECTORIES &&
+			test_expect_code 129 git -C sub $builtin -h >output 2>&1
+		) &&
 		test_i18ngrep usage output
 	'
 done <builtins