tests/test-parseindex2.py.out

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

done