.arcconfig

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

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