I've analysed multiple nigthly composes and often the only difference
in the configuration between two nightly composes is different
`product_id` commit hash.
The `product_id` is used in later `createrepo` phase and does not
influence the gather phase at all. I therefore think it can be
whitelisted in gather phase reuse code.
Signed-off-by: Jan Kaluza <jkaluza@redhat.com>