manifest: have context's dirs() call its manifest's dirs()

This lets the context's dirs() method be agnostic towards any alternate
manifest implementations.

mercurial/context.py

@@ -266,12 +266,9 b' class basectx(object):'
         diffopts = patch.diffopts(self._repo.ui, opts)
         return patch.diff(self._repo, ctx2, self, match=match, opts=diffopts)
 
-    @propertycache
-    def _dirs(self):
-        return scmutil.dirs(self._manifest)
+    def dirs(self):
+        return self._manifest.dirs()
 
-    def dirs(self):
-        return self._dirs
 
     def dirty(self, missing=False, merge=True, branch=True):
         return False