t5000: inspect HEAD using git-rev-parse

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Reviewed-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5000-tar-tree.sh

@@ -243,10 +243,11 @@ test_expect_success 'validate file modification time' '
 	test_cmp expected.mtime b.mtime
 '
 
-test_expect_success \
-    'git get-tar-commit-id' \
-    'git get-tar-commit-id <b.tar >b.commitid &&
-     test_cmp .git/$(git symbolic-ref HEAD) b.commitid'
+test_expect_success 'git get-tar-commit-id' '
+	git get-tar-commit-id <b.tar >actual &&
+	git rev-parse HEAD >expect &&
+	test_cmp expect actual
+'
 
 test_expect_success 'git archive with --output, override inferred format' '
 	git archive --format=tar --output=d4.zip HEAD &&