notmuch new: Restrict the "not much" pun to the first run.

Several people complained that the humor wore thin very quickly.  The
most significant case of "not much mail" is when counting the user's
initial mail collection. We've promised on the web page that no matter
how much mail the user has, notmuch will consider it to be "not much"
so let's say so. (This message was in place very early on, but was
inadvertently dropped at some point.)