emacs: new notmuch-tree-process-exit-functions

Hook run when the tree insertion process finishes its job.

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This patch supersedes <id:20220816214023.1523322-1-jao@gnu.org>, but
changing the new variable name.

Right now, it can be used for silly things like removing or changing
the the "End of search." hardcoded message in the tree buffer.  But
also for more sophisticated things like folding all threads in add-ons
like my outline mode for tree buffers (to be submitted).

Signed-off-by: jao <jao@gnu.org>