[MERGE] Improve warning to 'bzr upgrade' to specify --rich-root or --subtree if needed.

[Andrew Bennetts]

Andrew Bennetts wrote:
> This patch improves the warning you see when you use an old repository to
> suggest “bzr upgrade --rich-root” or “bzr upgrade --subtree” if necessary.  This

bb:resubmit

Jelmer pointed out to me that currently --rich-root is an alias for a knit-based
repository, rather than an alias for the most preferred format with rich roots.

So this patch as is isn't quite right.  Perhaps we should change the meaning of
--rich-root (and seeing as I had assumed it meant something else, that's
possibly a good idea).  Or perhaps this patch should be abandoned.

-Andrew.