Command.Add.seek starts concurrency with CommandStages. And for
Command.Sync, it needs TransferStages. So, to get both types of concurrency
for the two different parts, it either needs to change the type of
concurrency in between, or just call startConcurrency once for each.

It seems safe enough to call startConcurrency twice, because it does shut
down concurrency (mostly) at the end, and eg the old Annex.workers get
emptied.

Sponsored-by: unqueued on Patreon

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It was a mistake for mirror to support --json, but that happened long
ago and it doesn't seem worth removing. I doubt anyone will use it.

This ended up having an interface like sync, rather than like get/copy/drop.
That let it be implemented in terms of sync, which took a lot less code.
Also, it lets it handle many of the edge cases that sync does, such as
getting files that are not visible in a --hide-missing branch, and sending
files to exporttree remotes.

As well as being easier to implement, `git-annex satisfy myremote` makes
sense as it satisfies the preferred content settings of the remote.
`git-annex satisfy somefile` does not form a sentence that makes sense. So
while -C can be a little bit annoying, it still makes sense to have this
syntax.

Note that, while I initially thought this would also satisfy numcopies, it
does not. Arguably it ought to. But, sync does not send files in order to
satisfy numcopies, it only sends files to satisfy preferred content. And
it's important that this transfer the same files as sync does, because
it will probably be used in a workflow where the user sometimes syncs and
sometimes satisfies, and does not expect satisfy to do things that sync
would not do.

(Also opened a new bug that also affects sync et all, not only this command.)

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This was already possible, but it was rather hard to come up with the
complex shell command needed.

Note that the diff output starts with "diff a/... b/...".
I left off the "--git" because it's not a git format diff.

Anarcat fixed these in the news file, so transferred it over