It's not concurrent-output safe, and doesn't support
--json-error-messages.
Using Annex.makeRunner is a bit scary, because what if it's run in a
different thread from an active annex action? Normally the same Annex
state is not used concurrently in several threads, and it's not designed
to be fully concurrency safe. (Annex.Concurrent exists to deal with
that.) I think it will be ok in these simple cases though. Eg,
when buffering a warning message to json, Annex.changeState is used,
and it modifies the MVar in a concurrency safe way.
The only warningIO remaining is not a problem.
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