bup.client: fix freeze when suggest-index after finishing a full pack.

bup.client: fix freeze when suggest-index after finishing a full pack.

It was just rare enough to be hard to find: if you write an entire pack full
of stuff (1GB or more) and *then* trigger a suggest-index, the client would
freeze because it would send a send-index command without actually
suspending the receive-pack first.

The whole Client/PackWriter separation is pretty gross, so it's not terribly
surprising this would happen.

Add a unit test to detect this case if it ever happens in the future, for
what it's worth.