revlog: extend addgroup() with callback for duplicates The addgroup() interface currently doesn't allow the caller to keep track of duplicated nodes except by looking at the returned node list. Add an optional second callback for this purpose and change the return type to a boolean. This allows follow-up changes to use more efficient storage for the node list in places that are memory-sensitive. Differential Revision: https://phab.mercurial-scm.org/D9231