setdiscovery: always add exponential sample to the heads As explained in a previous changeset, prioritizing heads too much behaves pathologically when there are more heads than the sample size. To counter this, we always inject exponential samples before reducing to the sample size limit. This already show some benefit in the test themselves, but on a real-world example this moves my discovery for push to pathologically headed repo from 45 rounds to 17 of them. We should maybe ensure that at least 25% of the result sample is heads, but I think the random sampling will be fine in practice.