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

hgext/fsmonitor/__init__.py

@@ -273,7 +273,11 b' def overridewalk(orig, self, match, subr'
 
     matchfn = match.matchfn
     matchalways = match.always()
-    dmap = self._map._map
+    dmap = self._map
+    if util.safehasattr(dmap, '_map'):
+        # for better performance, directly access the inner dirstate map if the
+        # standard dirstate implementation is in use.
+        dmap = dmap._map
     nonnormalset = self._map.nonnormalset
 
     copymap = self._map.copymap