"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 <redacted>

Forgot to mention: the $DAYJOB case is the same case as in [1]. In [1] I
noticed that the object wasn't actually being used, so I disabled the
object existence check. But that's probably the wrong approach - if the
caller really didn't want the object's existence to be checked, they
could have used WRITE_TREE_MISSING_OK when calling cache_tree_update().

[1] https://lore.kernel.org/git/cover.1627066238.git.jonathantanmy@google.com/ (local)