filelog: declare that filelog implements a storage interface Now that we have a declared interface, let's declare that filelog implements it. Tests have been added that confirm the object conforms to the interface. The existing interface checks verify there are no extra public attributes outside the declared interface. filelog has several extra attributes. So we added a mechanism to suppress this check. The goal is to modify the filelog class so we can drop this check. Differential Revision: https://phab.mercurial-scm.org/D3149