emacs: run notmuch-search-hook lazily

emacs: run notmuch-search-hook lazily

In message id:YT3ueuZHKW931NW3@localhost, Fabio Natali isolated a
visual glitch caused by running notmuch-search-hook too early. This
change moves the running of that hook to
notmuch-search-process-filter, which ensures there is some output in
the buffer before running the hook. Since n-s-p-f can be called many
times for a given buffer, add a buffer local flag to make sure it is
only run once per buffer.