statichttp: add the missing `features` attribute

This make statichttp closer to the interface it is "supposed" to follow.

Differential Revision: https://phab.mercurial-scm.org/D10616

mercurial/statichttprepo.py

@@ -177,6 +177,7 b' class statichttprepository('
         self.filtername = None
         self._extrafilterid = None
         self._wanted_sidedata = set()
+        self.features = set()
 
         try:
             requirements = set(self.vfs.read(b'requires').splitlines())