fsmonitor: increase the threshold before we recommend it, when using rust 50k files works just fine with the rust status, and it's annoying to get told "consider using fsmonitor" when we have another solution to the status speed. 400k files was not chosen in any rigorous way. I know 200k files is fine. Twice as many files should still be fine. Differential Revision: https://phab.mercurial-scm.org/D9021