largefiles: prevent in-memory merge instead of switching to on-disk I enabled in-memory merge by default while testing some changes. I spent quite some time troubleshooting why largefiles was still creating an on-disk mergestate. Then I found out that it ignores the callers `wc` argument to `mergemod._update()` and always uses on-disk merge. This patch changes that so we raise an error if largefiles is used with in-memory merge. That way we'll notice if in-memory merge is used with largefiles instead of silently replacing ignoring the `overlayworkingctx` instance and updating the working copy instead. I felt a little bad that this would break things more for users with both largefiles and in-memory rebase enabled. So I also added a higher-level override to make sure that largefiles disables in-memory rebase. It turns out that that fixes `run-tests.py -k largefiles --extra-config-opt rebase.experimental.inmemory=1`. Differential Revision: https://phab.mercurial-scm.org/D9069