ostree-container: Add diffid as metadata to refs #2078
Description
(This issue would be obviated by #20 )
Today the ostree-container backend stores layers indexed by their manifest digest, but it would have made more sense to index by diffid.
But we can compatibly handle this today: before initiating a fetch, iterate over all stored ostree-container manifests, and build a mapping from blob -> diffid. If we go to fetch a layer, and it has a different blob but the same diffid, just reuse that existing layer. I think that should be it.
This should be relatively easy to integration test by constructing an oci: storage that has the same diffid, one layer compressed with gzip and another with zstd for example.