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