manifest: use read_any_fast_delta in changectx

The new API is clearer but also more expressive. It allow to detect case where
we did return a full read and populated the associated cache. Saving time!

mercurial/context.py

@@ -541,7 +541,10 b' class changectx(basectx):'
 
     @propertycache
     def _manifestdelta(self):
-        return self._manifestctx.readdelta()
+        base, delta = self._manifestctx.read_any_fast_delta()
+        if base is None:
+            self._manifest = delta
+        return delta
 
     @propertycache
     def _parents(self):