Update prefix so that "thread:" can be used in search strings.

It's convenient to be able to do things like:

     notmuch tag -inbox thread:<thread-id>

(even though this can run into a race condition as noted in TODO--the fix
for the race is simply to not run "notmuch new" between reading a thread
with the (not yet existent) "notmuch show" and removing its inbox tag
with a command like the above). So we now allow such a thing.

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