Code review comment for lp:~mbp/bzr/456077-cross-format-fetch

As a followon, when this merges to trunk, it can take https://bugs.edge.launchpad.net/bzr/+bug/507655 into account. In 2.1 and later we have a config.suppress_warnings option that will let you turn them on/off per location; presumably that should be checked before going into the uifactory.