This optimizes the user's tagging query to exclude messages that won't
be affected by the tagging operation, saving computation and IO for
redundant tagging operations.
For example,
notmuch tag +notmuch to:notmuch@notmuchmail.org
will now use the query
( to:notmuch@notmuchmail.org ) and (not tag:"notmuch")
In the past, we've often suggested that people do this exact
transformation by hand for slow tagging operations. This makes that
unnecessary.