statichttprepo: use in-place difference for shorter line

mercurial/statichttprepo.py

@@ -124,7 +124,7 b' class statichttprepository(localrepo.loc'
         self._branchcachetip = None
         self.encodepats = None
         self.decodepats = None
-        self.capabilities = self.capabilities.difference(["pushkey"])
+        self.capabilities.difference_update(["pushkey"])
 
     def url(self):
         return self._url