Add a script to quickly run an interactive test session.

This uses the capabilities of the test library to provide a throw-away
playground to test how the current stgit tree behaves.

To use it, run the following command:
	(cd t && ./interactive.sh)

You'll get a shell in a brand new git tree.  Exiting that shell will
get rid of this temporary directory.

Signed-off-by: Yann Dirson <ydirson@altern.org>