emacs: hello: use batch count

This modifies notmuch hello to use the new count --batch
functionality. It should give exactly the same results as before but
under many conditions it should be much faster. In particular it is
much faster for remote use.

The code is a little ugly as it has to do some working out of the
query when asking the query and some when dealing with the result.
However, the code path is exactly the same in both local and remote
use.