sha1_file: teach packed_object_info about typename

sha1_file: teach packed_object_info about typename

In commit 46f0344 ("sha1_file: support reading from a loose object of
unknown type", 2015-05-06), "struct object_info" gained a "typename"
field that could represent a type name from a loose object file, whether
valid or invalid, as opposed to the existing "typep" which could only
represent valid types. Some relatively complex manipulations were added
to avoid breaking packed_object_info() without modifying it, but it is
much easier to just teach packed_object_info() about the new field.
Therefore, teach packed_object_info() as described above.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>