repo_read_object_file(): stop wrapping read_object_file_extended()

repo_read_object_file(): stop wrapping read_object_file_extended()

The only caller of read_object_file_extended() is the thin wrapper of
repo_read_object_file(). Instead of wrapping, let's just rename the
inner function and let people call it directly. This cleans up the
namespace and reduces confusion.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>