merge: pass wctx to batchremove and batchget We would like to migrate direct calls of repo.wvfs/wwrite/wread/etc to a call on the relevant workingfilectx, both as a cleanup (to reduce the number of working copy functions on `repo`), and also to facilitate an in-memory merge that doesn't write to the working copy. In order to do that, the first step is to ensure we pass the target wctx around and perform our writes and reads on it. Later, this object might become a memctx.