Test suite: examplegit setup script

We are going to have `setup' scripts which explain how to set up the
preconditions for one or more tests.  This is the first one.

Right now it runs to completion, generating its deliverables in $tmp,
but then crashes due to the as-yet-unimplemented t-setup-done call.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>