contrib/subtree: Add tests for subtree add
contrib/subtree: Add tests for subtree add

Add some tests to check various options to subtree add.  These test
various combinations of --message, --prefix and --squash.

Signed-off-by: Techlive Zheng <e0b8d50107251974aaf777424740d7be1f7d7730@gmail.com>
Signed-off-by: David A. Greene <699889fa4d775c82b36574aaa0f8ebb772e0c366@obbligato.org>
Signed-off-by: Jeff King <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git