debugdiscovery: only list common heads on verbose The list of common heads is only part of the useful information. In addition on repository with many heads, the information is very not helpful (just fill a couple of screen with hash). As a result we hide it behind a --verbose flag.

debugdiscovery: drop duplicated information The old line informing about the local being a superset or subset of the remote is redundant with the newly introduced data. So we drop it.

debugdiscovery: display more statistic about the common set We display a lot more information now. Especially, we display the overlap between the common heads and the local/remote heads. There are various optimization geared toward heads, as a result, the less common the heads the more complex the discovery. Having this information easily accessible help when working on discovery.