manifest: define and implement rev() on manifestlog Various code is accessing repo.manifestlog._revlog - a private attribute. This bypasses our interface and makes it difficult to implement non-revlog manifest storage. This commit adds a rev() method to the manifestlog interface and class and teaches callers to use it. Ideally this method wouldn't exist, as very few consumers should need to resolve the revision number of a manifest. Again, the primary goal of interface work is to establish and use interfaces first and to improve them later. Differential Revision: https://phab.mercurial-scm.org/D3882