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

diff --cc diff.c
Simple merge

diff --cc object-store.h
Simple merge

diff --cc sha1-file.c
Simple merge

diff --cc unpack-trees.c
Simple merge