manifest: add manifestdict.hasdir() method

Allows for alternative implementations of manifestdict to decide if a directory
exists in whatever way is most optimal.

mercurial/manifest.py

@@ -155,6 +155,9 b' class manifestdict(object):'
     def dirs(self):
         return self._dirs
 
+    def hasdir(self, dir):
+        return dir in self._dirs
+
     def matches(self, match):
         '''generate a new manifest filtered by the match argument'''
         if match.always():