setdiscovery: randomly pick between heads and sample when taking full sample Before this changeset, the discovery protocol was too heads-centric. Heads of the undiscovered set were always sent for discovery and any room remaining in the sample were filled with exponential samples (and random ones if any room remained). This behaved extremely poorly when the number of heads exceeded the sample size, because we keep just asking about the existence of heads, then their direct parent and so on. As a result, the 'O(log(len(repo)))' discovery turns into a 'O(len(repo))' one. As a solution we take a random sample of the heads plus exponential samples. This way we ensure some exponential sampling is achieved, bringing back some logarithmic convergence of the discovery again. This patch only applies this principle in one place. More places will be updated in future patches. One test is impacted because the random sample happen to be different. By chance, it helps a bit in this case.