manifest: remove addgroup() from manifestlog and imanifestlog addgroup() is part of the storage interface for manifests. Unfortunately, we don't yet have a formal storage interface for manifests. (One will be established in subsequent commits.) One thing is for sure, addgroup() doesn't belong on imanifestlog - at least not unless we extend that interface to encompass storage. For now, let's access addgroup() on the _revlog attribute, just like we do for tree manifests. Even though this violates visibility, it is consistent. Differential Revision: https://phab.mercurial-scm.org/D4274