fsmonitor: only access inner dirstate map if it is available
As part of the dirstate refactor, fsmonitor was updated to directly access the
inner map of the dirstatemap object.
Dirstatemap reimplementations may not use a map like this, so only access it if
it is there.
Differential Revision: https://phab.mercurial-scm.org/D1346
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.