It is a common pattern to obtain a directory manifest storage instance
(a manifestrevlog) by going through manifestlog._revlog.dirlog().
Why access to storage and caching of other manifests is done through
manifestrevlog instead of manifestlog, I don't know.
This commit establishes a getstorage(tree) API on manifestlog and
imanifestlog that provides a public API for accessing manifest storage.
All consumers previously using private attributes have been updated
to use this new method.
.. api:: manifestlog now has a getstorage(tree) method
It should be used for obtaining an object representing the manifest's storage implementation. Accessing manifestlog._revlog should be avoided.