wireprotov2: teach changesetdata to fetch ancestors until depth For shallow clone, it is useful to specify a starting node and tell the server to send up to N ancestors from that starting point. This enables the server to perform the DAG walk without the client having to discover the base/stop node(s) first. This commit implements support for said queries on the changesetdata command. Differential Revision: https://phab.mercurial-scm.org/D4621