contrib/subtree: use tabs consitently for indentation in tests

Although subtrees tests uses more spaces for indentation than tabs,
there are still quite a lot of lines indented with tabs. As tabs conform
with Git coding guidelines resolve the inconsistency in favour of tabs.

Signed-off-by: Charles Bailey <cbailey32@bloomberg.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: hashpling