The only way that "obj" can be non-NULL is if it came from
one of the lookup_* functions. These functions always ensure
that the object has the expected type (and return NULL
otherwise), so there is no need for us to set the type.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>