[merge][0.15] recommend upgrading workingtrees

[Martin Pool]

This patch makes bzr suggest upgrading if you are using an old
workingtree format.  This is a good idea because the new format is
much faster, and the old xml formats have got slightly slower(?).

Example:

mbp at limpid% ./bzr st ../trunk
Working tree format 3 is deprecated and a better format is available.
It is recommended that you upgrade by running the command
  bzr upgrade file:///home/mbp/bzr/trunk/

This doesn't happen when the workingtree is opened by the init, info
or upgrade commands.

We could also add upgrade_recommended to the branch and repositories
but I haven't done that for now.

This is similar to, but different from, the existing facility for
formats that we can read but only for upgrade.

The idea of offering a way to suppress this per-path was floated; in
the end I couldn't convince myself that adding a special mechanism for
warning was a good idea.

This puts the actual representation of the message into the ui class.

-- 
Martin
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