When :undo! was introduced to Nvim the implementation of 'inccommand'
preview callback hasn't been fully decided yet, so not notifying buffer
update callbacks made sense for 'inccommand' preview callback in case it
needs to undo the changes itself.
Now it turns out that the undo-and-forget is done automatically for
'inccommand', so it doesn't make sense for :undo! to avoid notifying
buffer update callbacks anymore.
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