setdiscover: allow to ignore part of the local graph Currently, the push discovery first determines the full set of common nodes before looking into what changesets are outgoing. When pushing a specific subset, this can lead to pathological situations where we search for the status of thousand of local heads that are unrelated to the requested pushes. To fix this, we need to teach the discovery to ignores part of the graph. Most of the necessary pieces were already in place. This changeset just makes them available to higher level API and tests them. Change actually impacting pushes are coming in a later changeset.