bzr upgrade should choose a reasonable default format

As of bzr 1.5, there are some old formats (rich-root, subtree) that contain more information than the bzr default format and so cannot be upgraded to the current default format. With such a directory running just 'bzr upgrade' will give an error that the formats are incompatible.

I think upgrade should determine the best/newest recommended format that's compatible with the source.

This bug could also be avoided by making sure the current default format *does* support upgrades from every previous format. (But not really, as it's possible the user has a component created by a plugin, etc.)