cache-tree: prefetch in partial clone read-tree

cache-tree: prefetch in partial clone read-tree

"git read-tree" checks the existence of the blobs referenced by the
given tree, but does not bulk prefetch them. Add a bulk prefetch.

The lack of prefetch here was noticed at $DAYJOB during a merge
involving some specific commits, but I couldn't find a minimal merge
that didn't also trigger the prefetch in check_updates() in
unpack-trees.c (and in all these cases, the lack of prefetch in
cache-tree.c didn't matter because all the relevant blobs would have
already been prefetched by then). This is why I used read-tree here to
exercise this code path.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>