overlwayworkingctx: remove doubly bad reference to wrapped ctx for copies `_wrappedctx` lives on overlwayworkingctx, not on the repo object, so we should access it as `._wrappedctx`, not `._repo._wrappedctx`. More importantly, the overlayworkingctx is relative to its base, not including it, so the copies returned should not include copies made in the base. Differential Revision: https://phab.mercurial-scm.org/D7801