manifest: define and implement addgroup() on manifestlog Changegroup code was bypassing our manifest interface and calling a method on the private revlog attribute. This commit formalizes the interface for adding a group of revisions from deltas and changes the changegroup code to use it. This enables alternate manifest storage to work with changegroup application operations (like `hg unbundle` and `hg pull`). Differential Revision: https://phab.mercurial-scm.org/D3883