[merge][0.91] set default format to dirstate-tags

[Martin Pool]

This builds on my previous patch that adjusts append_revision and so on.

 * set the default branch format and bzrdir format -- the concrete
change caused by this is that we now use just a last-revision marker,
and have tags
 * remote branches now support tags -- of course whether it actually
works depends on the remote branch format

There is some divergence between formats about whether you are allowed
to point a branch to a revision that's not in its repository.  A few
methods assume that it is present and look at the graph.  The tests
that rely on this being possible seem to do so only accidentally, so I
have just adjusted them to use a revision that is present.

I'd like to merge this as soon as we branch off 0.90 so that we can
get the most testing with this format in place.

-- 
Martin
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