tree-walk: convert get_tree_entry_follow_symlinks to object_id

Since the only caller of this function already uses struct object_id,
update get_tree_entry_follow_symlinks to use it in parameters and
internally.

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