tests: add test for widening from an empty clone Narrow clones that track no paths currently don't even include the root manifest (which is the only manifest when using flat manifests). That means that when we widen from such a clone, we need to make sure that we send the root manifest (and other manifests if using tree manifests). That currently works because we always resend all manifest that match the new narrowspec. However, we're about to stop resending manifests that the client already has and there's a risk of this breaking then, so let's add a test. Differential Revision: https://phab.mercurial-scm.org/D5143