Using Logs.RemoteState for this means that if the same key gets uploaded
twice to a git-lfs remote, but somehow has different content the two
times (eg it's an URL key with non-stable content), the sha256/size of
the newer content uploaded will overwrite what was remembered before. That
seems ok; it just means that git-annex will request the newer version of
the content when downloading from git-lfs.
It will remember the sha256 and size if both are not known, or if only
the sha256 is not known but the size is known, it only remembers the
sha256, to avoid wasting space on the size. I did not add special case
for when the sha256 is known and the size is not, because it's been a
long time since git-annex created SHA256 keys without a size.
(See doc/upgrades/SHA_size.mdwn)
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