consolidate two copies of new style object header parsing code.

consolidate two copies of new style object header parsing code.

Also while we are at it, remove redundant typename[] array from
unpack_sha1_header.  The only reason it is different from the
type_names[] array in object.c module is that this code cares
about the subset of object types that are valid in a loose
object, so prepare a separate array of boolean that tells us
which types are valid, and share the name translation with the
others.

Signed-off-by: Junio C Hamano <junkio@cox.net>