repair: move manifest strip to a separate function This moves manifest stripping to a separate function so implementations of the manifest that don't support stripping can replace this function with a no-op. I considered adding a strip api to the manifestlog, so other implementations could make it a no-op there, but it seems like strip might be unique to the revlog implementation, and therefore shouldn't be present on the generic api. Differential Revision: https://phab.mercurial-scm.org/D292