diff-tree: avoid lookup_unknown_object
We generally want to avoid lookup_unknown_object, because it
results in allocating more memory for the object than may be
strictly necessary.
In this case, it is used to check whether we have an
already-parsed object before calling parse_object, to save
us from reading the object from disk. Using lookup_object
would be fine for that purpose, but we can take it a step
further. Since this code was written, parse_object already
learned the "check lookup_object" optimization, so we can
simply call parse_object directly.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>