allow repo/mirror IP addresses to come from other sources than DNS
Doing a DNS request for f-droid.org announces to the network that the client is trying to connect to f-droid.org, even if the client is only connecting to private mirrors. DNS request can often be in plain text, so anyone that can see the network traffic can see the request. The IP addresses for f-droid.org and mirrors rarely change, and there is already the index as an alternate channel, so it should be possible for fdroidclient to only use DNS when the IP addresses it already knows about do not work.