object-file: move empty_tree struct into find_cached_object()

object-file: move empty_tree struct into find_cached_object()

The fake empty_tree struct is a static global, but the only code that
looks at it is find_cached_object(). The struct itself is a little odd,
with an invalid "oid" field that is handled specially by that function.

Since it's really just an implementation detail, let's move it to a
static within the function. That future-proofs against other code trying
to use it and seeing the weird oid value.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>