Compare types in test_orig_head with "is"

This performs an exact type comparison with "is" instead of "==".
Most (maybe all?) of the rest of the places where "==" was used
have been changed previously, but this on fell through the cracks.
Like the None object, type objects use reference-based equality
comparison, and the idiomatic ways to check against them are "is"
for exactly comparisons and isinstance/issubclass otherwise.

test/test_refs.py

@@ -207,7 +207,7 @@ def test_is_valid(self):
         assert not SymbolicReference(self.rorepo, "hellothere").is_valid()
 
     def test_orig_head(self):
-        assert type(self.rorepo.head.orig_head()) == SymbolicReference
+        assert type(self.rorepo.head.orig_head()) is SymbolicReference
 
     @with_rw_repo("0.1.6")
     def test_head_checkout_detached_head(self, rw_repo):