tree-walk: convert fill_tree_descriptor() to object_id

tree-walk: convert fill_tree_descriptor() to object_id

All callers of fill_tree_descriptor() have been converted to object_id
already, so convert that function as well.  As a nice side-effect we get
rid of NULL checks in tree-diff.c, as fill_tree_descriptor() already
does them for us.

Helped-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Reviewed-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>