lfs: fix a discrepancy with a function wanting a filelog, but calling it rlog This conceptually broke in 1541e1a8e87d when the filelog isa revlog relationship was changed to containment of the revlog. It was made more obvious in 62a532045e71 and related API simplification. It's resolved in favor of passing a revlog because the revlog verification code doesn't have a reference to a filelog. Differential Revision: https://phab.mercurial-scm.org/D7711