Checksum as content is received from a remote git-annex repository, rather
than doing it in a second pass.
Not tested at all yet, but I imagine it will work!
Not implemented for any special remotes, and also not implemented for
copies from local remotes. It may be that, for local remotes, it will
suffice to use rsync, rely on its checksumming, and simply return Verified.
(It would still make a checksumming pass when cp is used for COW, I guess.)
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