support git-annex-lookupkey

[thkoch2001]

Instead of relying on parsing the key from the blob, just use git-annex.

[alpernebbi]

I'm assuming you're referring to the part in AnnexedFileTree. I can't use lookupkey there because the file tree might not exist on-disk, and there isn't a lookupkey --branch=treeish option like e.g. find does.

Even if that's added, I expect doing it in python would be faster until we have a lookupkey --batch --json that can take not only {"file":"path"} but {"branch":"ref", "file":"path"}, as we would have to run a process for every lookup otherwise (or at best for every tree object with a lookupkey --batch --branch=treeish) .