narrow: overwrite readfast in excludeddirmanifestctx

The excludeddirmanifestctx does not have underlying storage (because it is
excluded) so the implementation of the `readfast` trying to access it crash.
This was reveled while running some cache warning code on narrow repositories.
That code will be introduced further down in the series.

Differential Revision: https://phab.mercurial-scm.org/D9731

mercurial/manifest.py

@@ -2297,6 +2297,10 b' class excludeddirmanifestctx(treemanifes'
     def read(self):
         return excludeddir(self._dir, self._node)
 
+    def readfast(self, shallow=False):
+        # special version of readfast since we don't have underlying storage
+        return self.read()
+
     def write(self, *args):
         raise error.ProgrammingError(
             b'attempt to write manifest from excluded dir %s' % self._dir