narrow: open the transaction sooner when unbundling

That way, the narrow spec changes will be done within a transaction.

hgext/narrow/narrowbundle2.py

@@ -261,6 +261,7 b' def _handlechangespec(op, inpart):'
     # other servers may include a changespec part even when not widening (e.g.
     # because we're deepening a shallow repo).
     if util.safehasattr(repo, 'setnewnarrowpats'):
+        op.gettransaction()
         repo.setnewnarrowpats()