object: apply skip_hash and discard_tree optimizations to unknown blobs too

object: apply skip_hash and discard_tree optimizations to unknown blobs too

parse_object_with_flags() has an optimization to skip parsing blobs if
PARSE_OBJECT_SKIP_HASH_CHECK is set and the object hasn't been seen
before or might be a blob but hasn't been parsed yet. The latter can
happen, for example, if add_tree_entries() walks a path that references
a blob object that hasn't been seen before: lookup_blob() marks the
referenced oid as being a blob, but does not provide any additional
information about it until it is parsed.

It's possible for an object to be created without even a type, such as
when prepare_revision_walk() uses mark_uninteresting() to mark all
promisor objects as uninteresting. These objects have obj->parsed ==
false and obj->type == OBJ_NONE.

The skip_hash optimization does not consider this kind of object, so
parse_object_with_flags() proceeds to fully parse the object to
determine its type.

Improve the optimization by applying it to OBJ_NONE objects as well as
OBJ_BLOB ones. Apply a similar fix for trees.

Fixes: 8db2dad7a045 ("parse_object(): check on-disk type of suspected blob")
Signed-off-by: Aaron Plattner <aplattner@nvidia.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>