treemanifest: disable readdelta optimization

When tree manifests are stored with one revlog per directory and
loaded lazily, it's unclear how much readdelta will help. If only a
few files change, then only a small part of the full manifest will be
loaded, and the delta chains should also be shorter for tree
manifests. Therefore, let's disable readdelta for tree manifests for
now.

mercurial/manifest.py

@@ -687,7 +687,7 @@ class manifest(revlog.revlog):
         return md
 
     def readdelta(self, node):
-        if self._usemanifestv2:
+        if self._usemanifestv2 or self._usetreemanifest:
             return self._slowreaddelta(node)
         r = self.rev(node)
         d = mdiff.patchtext(self.revdiff(self.deltaparent(r), r))