discovery: introduce a partialdiscovery object This object will ultimately gather the data about common, undecided and missing revs in a single place and deal with most graph related computations. The goal is both to clarify the algorithm and to help provides a simple and clear API that can be reimplemented in Rust. For now, we only moved the `common` set in the object. In this commit, some direct access to the "private" `disco._common` attribute persist. They have not been removed yet because we won't need to expose a full API identical to `incrementalmissingancestors` and it seems simpler to access the attribute directly until the replacement is in place.