Merge branch 'jt/bulk-prefetch'

"git read-tree" had a codepath where blobs are fetched one-by-one
from the promisor remote, which has been corrected to fetch in bulk.

* jt/bulk-prefetch:
  cache-tree: prefetch in partial clone read-tree
  unpack-trees: refactor prefetching code

diff --cc cache.h
Simple merge

diff --cc read-cache.c
Simple merge

diff --cc unpack-trees.c
Simple merge