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

[Andrew Bennetts]

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
would have helped Russel Winder, who got bitten by doing what the warning said
only to find that “bzr upgrade” failed.

To support this, I added a “supports_tree_reference” method to Repository (with
trivial per-implementation test), to match the supports_rich_root method it
already has.  This wasn't strictly necessary, but I expect it will be useful for
RemoteRepository sooner or later (for RemoteRepository it's much easier if
format features are checked via the repository instance than via a format).

I also added a hidden format called “subtree”, so that bzr upgrade --subtree
would work.  It's an alias for pack-0.92-subtree at the moment.  I think this is
a reasonable alias to have, after all we already have --rich-root.

This is related to <https://bugs.launchpad.net/bzr/+bug/252908>.  It doesn't fix
that bug, but it does make it less urgent.

One nice thing about extending the warning message this way is that it is a
reminder to the user that their repository is not in a default format.  So I
think this is a good improvement to have, even if we do fix 252908 soon as well.

-Andrew.

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