Merge branch 'jt/batch-fetch-blobs-in-diff'

While running "git diff" in a lazy clone, we can upfront know which missing blobs we will need, instead of waiting for the on-demand machinery to discover them one by one. Aim to achieve better performance by batching the request for these promised blobs. * jt/batch-fetch-blobs-in-diff: diff: batch fetching of missing blobs sha1-file: support OBJECT_INFO_FOR_PREFETCH