Refactor cooked_index_shard::handle_gnat_encoded_entry

This changes cooked_index_shard::handle_gnat_encoded_entry to modify
the incoming entry itself, and to return void rather than a new name.
this simplifies the caller a little, which is convenient for a
different series I am working on.

Approved-By: Tom de Vries <tdevries@suse.de>