contrib/subtree: Make each test self-contained

Each test runs a full repository creation and any subtree actions
needed to perform the test.  Each test starts with a clean slate,
making debugging and post-mortem analysis much easier.

Signed-off-by: Techlive Zheng <techlivezheng@gmail.com>
Signed-off-by: David A. Greene <greened@obbligato.org>
Signed-off-by: Jeff King <peff@peff.net>