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