--batch combined with -J now runs batch requests concurrently for many
commands. Before, the combination was accepted, but did not enable
concurrency. Since the output of batch requests can be in any order, --json
with the new "input" field is recommended to be used, to determine which
batch request each response corresponds to.
If --json is not used, batch mode still runs concurrently, using the usual
concurrent-output. That will not be very useful for most batch mode users,
probably, but who knows.
If a program was using --batch -J before, and was parsing non-json output,
this could break it. But, it was relying on git-annex not supporting
concurrency despite it being enabled, so it should have expected concurrent
output. So, I think that's ok.
annex.jobs does not enable concurrency in --batch mode, because that would
confuse programs that use --batch but don't expect concurrency.
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