fsmonitor: use nonnormalset from dirstatemap
`dirstate._nonnormalset` has been moved to `dirstate._map.nonnormalset` by
60927b19ed65 (dirstate: move nonnormal and otherparent sets to dirstatemap)
and is guaranteed to be existed.
Let's update fsmonitor code to use the new `nonnormalset`. Thix fixed a perf
regression that slows down `hg status` by 0.5 seconds in one of our
production repos.
Differential Revision: https://phab.mercurial-scm.org/D1184
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