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.

debugdiscovery: small internal refactoring The part of the code displaying statistic is made independant from the one running the discovery. In the same do, the declaration of the discovery function is a bit simplified.

debugdiscovery: allow to select random seed during debugdiscovery run The randomness can lead to large timing difference, controling it is important.

discovery: move cl.hasnode outside of the for-loop IIUC, resolving attributes for changelog can lead to some overhead. So this patch moves that to outside of a for-loop. Differential Revision: https://phab.mercurial-scm.org/D6147

discovery: prevent deleting items from a dictionary Removing elements from Python dictionary is expensive. So let's prevent adding them instead. I added a newline to make code look a bit better. Differential Revision: https://phab.mercurial-scm.org/D6146

discovery: drop some unused sets Differential Revision: https://phab.mercurial-scm.org/D6145