repository: teach addgroup() to receive data with missing parents The way the narrow extension works today, the server rewrites outgoing changegroup data to lie about parents when the parents data is missing. It adds the ellipsis flag to the revision so it can be recorded as such in the revlog. In the new wire protocol, such rewriting does not occur on the server (at least not yet anyway). Instead, it is up to the client to recognize when it has received a revision without its parents. This means rewriting will be performed on the client. Furthermore, the mechanism for storing a shallow revision may differ from store to store. For example, the revlog store uses the ellipsis flag to denote a revision's parents have been rewritten. But a non-revlog store may wish to store things differently. And, some stores may not even support receiving shallow revision data! Therefore, it makes sense for the store itself to be making decisions about what to do when they receive revision data without their parents. This commit teaches the addgroup() bulk insert method to accept a boolean argument that indicates whether the incoming data may lack parent revisions. This flag can be set when receiving "shallow" data from a remote. The revlog implementation of this method has been taught to rewrite the missing parent(s) to nullid and to add the ellipsis flag to the revision when a missing parent is encountered. But it only does this if ellipsis flags are enabled on the repo and the incoming data is marked as possibly shallow. An error occurs otherwise. Differential Revision: https://phab.mercurial-scm.org/D5165