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 <[email protected]>
Signed-off-by: David A. Greene <[email protected]>
Signed-off-by: Jeff King <[email protected]>

Author: weynhamz