Fix count/search query destructiveness

[David Bremner]

David Bremner <david at tethera.net> writes:

> As discussed in id:147263183913.27784.12274024193186585889 at mbp, the
> current behaviour is destructive due to exclude handling. This makes
> some natural code, including notmuch-reply, buggy.

Mark points out on IRC that notmuch-reply doesn't use excludes, so it's
not buggy just confusing. So the current documentation should be
something like "the query functions are non-destructive, unless you use
excludes".  That makes these changes somewhat less urgent; I still think
they're probably a good idea, since the current is not very nice, and a
bit hard to document/understand.