tree-walk: copy object ID before use

tree-walk: copy object ID before use

In a future commit, the pointer returned by tree_entry_extract will
point into the struct tree_desc, causing its lifetime to be bound to
that of the struct tree_desc itself.  To ensure this code path keeps
working, copy the object_id into a local variable so that it lives long
enough.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>