From the commit message:
emacs: Make queries used in the all-tags section configurable
This patch adds a customization variable that controls what queries
are used to construct the all-tags section in notmuch-hello. It allows
the user to specify a function to construct the query given a tag. It
also allows hiding tags by returning nil.
Possible uses would be things like displaying the unread count for
tags instead of all messages with that tag and/or hiding uninteresting
tags like "signed" or "encrypted".
-- Daniel