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
$ hg init
$ echo a > a
$ hg ci -Am t
adding a
$ hg mv a b
$ hg ci -Am t1
$ hg debugrename b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3
$ hg mv b a
$ hg ci -Am t2
$ hg debugrename a
a renamed from b:37d9b5d994eab34eda9c16b195ace52c7b129980
$ hg debugrename --rev 1 b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3