Revert extra copying in Connection._rollback().

Since TmpStore.reset() had to change too to copy the index anyway,
it no longer hurts that _rollback() clears the index (as a side
effect of self._cache.invalidate(src.index)).

src/ZODB/Connection.py

@@ -1043,13 +1043,7 @@ class Connection(ExportImport, object):
         self._abort()
         self._registered_objects = []
         src = self._storage
-        # Caution:  it's possible that src.index is part of `state`, and
-        # invalidate() clears the container passed to it.  When rollback() is
-        # called on the same savepoint more than once, that's even likely.
-        # Therefore we must pass a copy of the oids to invalidate, lest
-        # we next reset `src` to have an empty index:  the ".keys()" here
-        # is vital.
-        self._cache.invalidate(src.index.keys())
+        self._cache.invalidate(src.index)
         src.reset(*state)
 
     def _commit_savepoint(self, transaction):