notmuch: use global settings for vfolder-from-query

notmuch: use global settings for vfolder-from-query

When the <vfolder-from-query> command is run while already in a vfolder,
it takes the current vfolder as context.  That means the global settings
(nm_query_type, nm_db_limit, nm_default_uri) are ignored in favor of the
current vfolder's settings.  This is confusing behavior because the new
vfolder is otherwise independent from the old one.

Set the nm_uri_from_query() context to NULL so that it uses the global
settings instead.