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
#require test-repo jshint hg10
$ . "$TESTDIR/helpers-testrepo.sh"
run jshint on all tracked files ending in .js except vendored dependencies
$ cd "`dirname "$TESTDIR"`"
$ testrepohg locate 'set:**.js' \
> -X mercurial/templates/static/excanvas.js \
> 2>/dev/null \
> | xargs jshint