emacs: remove newlines from input to notmuch count --batch

Since a newline starts a new query in batch mode, this causes
mysterious crashes in the emacs interface if saved searches contain
newlines.  See the discussion at

      id:87wqhcxb5j.fsf@maritornes.cs.unb.ca

In general newlines seem to be just whitespace to the xapian query
parser, so this should be mainly harmless.